[CI] Fix test_macos with recent bash for nix-shell

[straight-shoota]

Fixes the original issue of #10110 by installing bash from brew:

2. The actual error is /nix/store/r0n4qzr800498l23cq80mg0k03j3aqms-cctools-binutils-darwin-wrapper-927.0.2/nix-support/setup-hook: line 138: ${role_pre}${cmd^^}=${cmd}: bad substitution. Looks like macos ships a really old bash version (3.2) and the substitution requires at least 4.0. So that should be easy to fix, just install a recent bash.

Also moves nix initialization to prepare_build stage.

[bcardiff]

I've been using GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin18) without any issues.

But if setting up a new bash on the CI is needed it fine.

I played a bit locally:

$ nix-shell --pure --run 'A=3 exit $A'
$ echo $?
0 # :-(

$ nix-shell --pure --run 'export A=3; exit $A'
$ echo $?
3 # :-)

So, maybe we need to change:

--- on_nix_shell nix-shell --pure $CI_NIX_SHELL_ARGS --run "'TZ=$TZ $command'"
+++ on_nix_shell nix-shell --pure $CI_NIX_SHELL_ARGS --run "'export TZ=$TZ; $command'"

wdyt?

[straight-shoota]

Adding export doesn't seem to help: https://github.com/straight-shoota/crystal/runs/2071013793 (straight-shoota@8f61287).

[straight-shoota]

Can this get an approval? This is necessary to make CI (somewhat) reliable on macos again.

[sdogruyol]

Thank you @straight-shoota 🙏