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