install_pants_from_pex env var setup leads to PEX_ROOT leak

[cburroughs]

See pantsbuild/pants#23129 (comment) and https://github.com/pantsbuild/scie-pants/pull/226/changes#r2892914835

[jsirois]

Actually, see: https://github.com/jsirois/pants-issues-23129

I fully worked this example and there are 2 scie-pants bugs. Once you fix the env={"PEX_TOOLS": "1"} bug, you find PEX_ROOT is still not set, despite what the lift manifest appears to say. If you dig deeper, you find the bad tools.pex PEX build.

So:

1. Needs env={**os.environ, "PEX_TOOLS": "1"} instead:

   scie-pants/tools/src/scie_pants/install_pants.py

   Line 108 in 3203faa

   env={"PEX_TOOLS": "1"},

2. Needs --no-strip-pex-env:

   scie-pants/package/src/tools_pex.rs

   Lines 80 to 107 in 3203faa

   	build_step!("Building the scie_pants `tools.pex`");
   	let tools_src_path = tools_path.join("src");
   	let tools_src = path_as_str(&tools_src_path)?;
   	let tools_pex_path = build_context.cargo_output_root.join("tools.pex");
   	let tools_pex = path_as_str(&tools_pex_path)?;
   	execute(
   	Command::new(&pbt_exe).args(
   	[
   	"pex",
   	"--disable-cache",
   	"--no-emit-warnings",
   	"--lock",
   	lock,
   	"-r",
   	requirements,
   	"-c",
   	"conscript",
   	"-o",
   	tools_pex,
   	"--venv",
   	"prepend",
   	"-D",
   	tools_src,
   	]
   	.iter()
   	.chain(interpreter_constraints.iter()),
   	),
   	)?;