rule runner breaks if there are not defined, which can happen due to how pants isolates test env

[asherf]

private link: https://app.circleci.com/pipelines/github/toolchainlabs/toolchain/37523/workflows/8b9bf63f-aaf9-4cb3-8945-f2aacce8cbc1/jobs/122624/tests#failed-test-0

also tried to add those env variables to [subprocess-environment].env_vars but it didn't seem to help.
related to #13046
@stuhood

[asherf]

actually nvm, modifying pants.toml ([subprocess-environment].env_vars) fixes the issue.
so @stuhood feel free to close this PR.

[stuhood]

Mm: this is reasonable to land: thanks. This bit was really just intended as defense for our own usage, so not executing it outside of this repository is reasonable.

FWIW: the EXECUTION_SLOT is set dynamically by pytest in pantsbuild/pants due to: https://github.com/pantsbuild/pants/blob/main/pants.toml#L109. And HOME is passed through via:

pants/pants.toml

Lines 111 to 121 in 5086a78

	[test]
	extra_env_vars = [
	# TODO: These are exposed to tests in order to allow for python interpreter discovery when
	# Pants-tests-Pants: in particular, the [python-setup] subsystem consumes them.
	# see https://github.com/pantsbuild/pants/issues/11638
	"PYENV_ROOT",
	"HOME",
	"PATH",
	# We'd always like complete backtraces in tests.
	"RUST_BACKTRACE=1",
	]