fix(bootstrap): handle when runfiles env vars don't point to current binary's runfiles

[rickeylev]

The stage1 bootstrap script had a bug in the find_runfiles_root
function where it would unconditionally use the RUNFILES_DIR et al
environment variables if they were set.

This failed in a particular nested context: an outer binary
calling an inner binary when the inner binary isn't a data
dependency of the outer binary (i.e. the outer doesn't contain
the inner in runfiles). This would cause the inner binary to
incorrectly resolve its runfiles, leading to failures. Such
a case can occur if a genrule calls the outer binary, which has
the inner binary passed as an arg.

This change adds a check to validate that the script's entry point
exists within the inherited RUNFILES_DIR before using it. If the
entry point is not found, it proceeds with other runfiles discovery
methods. This matches the system_python runfiles discovery logic.

Fixes #3187

[alexeagle]

@rickeylev is there a plan to turn on --@rules_python//python/config_settings:bootstrap_impl=script as default again?

[rickeylev]

Eventually, yes. IIRC, the main blocker is Bazel 8+ is required for rules_pkg to be able to process py_binary correctly. #2521 is the tracking issue

[alexeagle]

@rickeylev do you think the default could vary based on the Bazel version? Or maybe https://github.com/bazel-contrib/bazelrc-preset.bzl should set it?

[aignas]

We technically can set a different default and it may be a good idea, that said, I wonder if we should wait for the packaging rules to properly support symlinks before we do the flip.