Interactive PEX execution leaks PEX_ROOT at $BUILD_ROOT/.cache/pex_root.

[jsirois]

For example:

$ rm -rf .cache
$ ./pants run testprojects/src/python/hello/main/main.py 
09:27:23.34 [INFO] Completed: Building main.pex
09:27:24.12 [INFO] Completed: Building requirements.pex with 1 requirement: ansicolors==1.1.8
Hello, world!
$ tree .cache/
.cache/
└── pex_root
    ├── code
    │   └── da39a3ee5e6b4b0d3255bfef95601890afd80709
    │       ├── __main__.py
    │       └── PEX-INFO
    └── installed_wheels
        └── c7b5d77e89855f9b02c99e84cd863c5bdc2be329
            └── ansicolors-1.1.8-py2.py3-none-any.whl
                ├── ansicolors-1.1.8.dist-info
                │   ├── DESCRIPTION.rst
                │   ├── INSTALLER
                │   ├── METADATA
                │   ├── metadata.json
                │   ├── RECORD
                │   ├── REQUESTED
                │   ├── top_level.txt
                │   └── WHEEL
                └── colors
                    ├── colors.py
                    ├── csscolors.py
                    ├── __init__.py
                    ├── __pycache__
                    │   ├── colors.cpython-37.pyc
                    │   ├── colors.cpython-38.pyc
                    │   ├── csscolors.cpython-37.pyc
                    │   ├── csscolors.cpython-38.pyc
                    │   ├── __init__.cpython-37.pyc
                    │   ├── __init__.cpython-38.pyc
                    │   ├── version.cpython-37.pyc
                    │   └── version.cpython-38.pyc
                    └── version.py

9 directories, 22 files

The issue here is that PexEnvironment assumes it's only used by a Process:

pants/src/python/pants/backend/python/util_rules/pex_environment.py

Lines 121 to 143 in 4210c6c

	def environment_dict(self, *, python_configured: bool) -> Mapping[str, str]:
	"""The environment to use for running anything with PEX.
	If the Process is run with a pre-selected Python interpreter, set `python_configured=True`
	to avoid PEX from trying to find a new interpreter.
	"""
	d = dict(
	PATH=create_path_env_var(self.path),
	PEX_INHERIT_PATH="false",
	PEX_IGNORE_RCFILES="true",
	PEX_ROOT=str(self.pex_root),
	**self.subprocess_environment_dict,
	)
	# NB: We only set `PEX_PYTHON_PATH` if the Python interpreter has not already been
	# pre-selected by Pants. Otherwise, Pex would inadvertently try to find another interpreter
	# when running PEXes. (Creating a PEX will ignore this env var in favor of `--python-path`.)
	if not python_configured:
	d["PEX_PYTHON_PATH"] = create_path_env_var(self.interpreter_search_paths)
	return d
	@property
	def pex_root(self) -> PurePath:
	return PurePath(".cache/pex_root")

In the InteractiveProcess case when run_in_workspace = True (the case for the run and repl goals) the assumptions of PexEnvironment are wrong.