Previously we used PATH to steer interpreter selection for hermetic PEX
bootstrap and runtime. This led to fragile or impossible to solve
setups.

Switch to using PEX_PYTHON_PATH to both discover a bootstrap
interpreter and steer runtime interpreter selection. Introduce a
find_binary rule to facilitate this that uses ~portable bash for now.
Also introduce a PexRuntimeEnvironment to steer bootstrap interpreter
probing and allow specification of the PATH that should be exposed to
the PEX runtime environment.

Fixes pantsbuild#9760

# Rust tests and lints will be skipped. Delete if not intended.
[ci skip-rust]

# Building wheels and fs_util will be skipped. Delete if not intended.
[ci skip-build-wheels]

# Rust tests and lints will be skipped. Delete if not intended.
[ci skip-rust]

# Building wheels and fs_util will be skipped. Delete if not intended.
[ci skip-build-wheels]

…_search_path/dont_stomp_PATH

# Conflicts:
#	src/python/pants/backend/awslambda/python/awslambda_python_rules.py
#	src/python/pants/backend/python/lint/bandit/rules.py
#	src/python/pants/backend/python/lint/black/rules.py
#	src/python/pants/backend/python/lint/docformatter/rules.py
#	src/python/pants/backend/python/lint/flake8/rules.py
#	src/python/pants/backend/python/lint/isort/rules.py
#	src/python/pants/backend/python/lint/pylint/rules.py
#	src/python/pants/backend/python/rules/coverage.py
#	src/python/pants/backend/python/rules/download_pex_bin.py
#	src/python/pants/backend/python/rules/hermetic_pex.py
#	src/python/pants/backend/python/rules/pex.py
#	src/python/pants/backend/python/rules/pytest_runner.py
#	src/python/pants/backend/python/rules/run_setup_py.py
#	src/python/pants/backend/python/typecheck/mypy/rules.py
#	src/python/pants/engine/process.py

[ci skip-rust]
[ci skip-build-wheels]

[ci skip-rust]
[ci skip-build-wheels]

[ci skip-rust]
[ci skip-build-wheels]

[ci skip-rust]
[ci skip-build-wheels]

…_search_path/dont_stomp_PATH

[ci skip-rust]
[ci skip-build-wheels]

[ci skip-rust]
[ci skip-build-wheels]

…_search_path/dont_stomp_PATH

This will be useful in other contexts too.

[ci skip-rust]
[ci skip-build-wheels]