fix: avoid stage1 bootstrap stdlib shadowing (#3854)

The stage 1 system-python bootstrap imports standard-library modules
before it
re-execs into the configured runtime. If the target output directory
contains
files named like standard-library modules, such as `shutil.py` or
`types.py`,
the bootstrap can import those files instead and fail before user code
starts.

This change removes Python's unsafe script-directory prepend before
those early
imports, while preserving `sys.path[0]` when Python has already
suppressed that
prepend through `-P`/`PYTHONSAFEPATH` or isolated mode. Stage 2 now uses
the same
isolated-mode guard when removing or recreating the main-directory path.

It also pins the Windows venv entry point template to LF line endings so
the
compile-pip CI dirty-worktree check is stable across checkout settings.

Before this change, a generated target output could shadow bootstrap
stdlib
imports, and isolated-mode runs on older Python versions could lose a
required
stdlib path.

After this change, bootstrap stdlib imports resolve from the
interpreter's
standard library, isolated mode preserves the interpreter-provided path,
and the
line-ending-sensitive CI check stays clean.

Tests:

```shell
bazel test --config=fast-tests \
  //tests/bootstrap_impls:stdlib_shadowing_system_python_test

bazel test --config=fast-tests \
  //tests/bootstrap_impls:stdlib_shadowing_system_python_test \
  //tests/bootstrap_impls:interpreter_args_test \
  //tests/bootstrap_impls:sys_path_order_bootstrap_script_test
```

(cherry picked from commit fbb2ccf4ca4ac5d43f2e4181469b04dc14cfb092)