Cross compiling with virtual packages

[isuruf]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

Building on a x86 Linux machine for x86 windows fails.

$ cat meta.yaml 
package:
  name: foo
  version: 0.0.1

build:
  number: 0
  script:
    - echo 'hello'
    - ls $PREFIX
    # this is the unix layout.
    - test ! -d $PREFIX/x86_64-w64-mingw32
    - test -d $PREFIX/Library

requirements:
  build:
  host:
    - m2w64-sysroot_win-64
  run:

$ cat conda_build_config.yaml 
target_platform:
  - win-64

Running

conda build . -c conda-forge/label/m2w64-experimental -c conda-forge

fails the test, because it uses the unix noarch package instead of the windows noarch package.

cc @mbargull @jaimergp

Conda Info

active environment : base
    active env location : /home/isuru/miniforge3
            shell level : 1
       user config file : /home/isuru/.condarc
 populated config files : /home/isuru/miniforge3/.condarc
                          /home/isuru/.condarc
          conda version : 24.5.0
    conda-build version : 24.5.0
         python version : 3.10.13.final.0
                 solver : libmamba (default)
       virtual packages : __archspec=1=zen3
                          __conda=24.5.0=0
                          __glibc=2.35=0
                          __linux=6.5.0=0
                          __unix=0=0
       base environment : /home/isuru/miniforge3  (writable)
      conda av data dir : /home/isuru/miniforge3/etc/conda
  conda av metadata url : None
           channel URLs : https://conda.anaconda.org/conda-forge/linux-64
                          https://conda.anaconda.org/conda-forge/noarch
          package cache : /home/isuru/miniforge3/pkgs
                          /home/isuru/.conda/pkgs
       envs directories : /home/isuru/miniforge3/envs
                          /home/isuru/.conda/envs
               platform : linux-64
             user-agent : conda/24.5.0 requests/2.31.0 CPython/3.10.13 Linux/6.5.0-28-generic ubuntu/22.04.4 glibc/2.35 solver/libmamba conda-libmamba-solver/24.1.0 libmambapy/1.5.7
                UID:GID : 1000:1000
             netrc file : /home/isuru/.netrc
           offline mode : False

Conda Config

==> /home/isuru/miniforge3/.condarc <==
channels:
  - conda-forge
solver: libmamba

==> /home/isuru/.condarc <==
channels:
  - conda-forge
report_errors: False
solver: libmamba
conda_build:
  pkg_format: 2

Conda list

$ conda list --show-channel-urls | grep "^conda"
conda                     24.5.0          py310hff52083_0    conda-forge
conda-build               24.5.0          py310hff52083_1    conda-forge
conda-content-trust       0.2.0              pyhd8ed1ab_0    conda-forge
conda-forge-metadata      0.5.3                    pypi_0    pypi
conda-forge-pinning       2023.11.07.18.09.01      hd8ed1ab_0    conda-forge
conda-index               0.4.0              pyhd8ed1ab_0    conda-forge
conda-libmamba-solver     24.1.0             pyhd8ed1ab_0    conda-forge
conda-lock                2.4.2              pyhd8ed1ab_0    conda-forge
conda-oci-mirror          0.1.0                    pypi_0    pypi
conda-pack                0.7.1              pyhd8ed1ab_0    conda-forge
conda-package-handling    2.2.0              pyh38be061_0    conda-forge
conda-package-streaming   0.9.0              pyhd8ed1ab_0    conda-forge
conda-smithy              3.35.0             pyhd8ed1ab_0    conda-forge
conda-souschef            2.2.3              pyhd8ed1ab_0    conda-forge
conda-suggest             0.1.1              pyh9f0ad1d_0    conda-forge
conda-suggest-conda-forge 2021.8.24            ha770c72_0    conda-forge
conda-tree                1.1.0              pyhd8ed1ab_2    conda-forge

Additional Context

No response

[mbargull]

I took a quick look:
Fixing this should be fairly simple with just

     with env_vars(
         {
             "CONDA_ALLOW_NON_CHANNEL_URLS": "true",
             "CONDA_SOLVER_IGNORE_TIMESTAMPS": "false",
+            "CONDA_SUBDIR": subdir,
         },
         callback=reset_context,
     ):

at https://github.com/conda/conda-build/blob/24.5.0/conda_build/environ.py#L1263-L1269 and passing subdir on to install_actions from get_install_actions/get_package_records, i.e., adding a simple test would be most of the work.
(Yeah, that's an unexpectedly obvious oversight on our part, whoops. Should also be fixed when gh-5154 gets tackled.)

As for when/how to include this:

1. We could include this in a 24.5.x patch release ("fixes cross-platform building with virtual packages"),
2. or include this only in 24.7.x ("adding support for proper cross-platform building").

In principle, I'd be in favor of option 1, but am also not sure if this could be an "unexpected environment resolving change for a patch release" for some.

[jaimergp]

Added #5350 with a test