fix: add runfiles root for system_python bootstrap

[rickeylev]

When the system_bootstrap code was changed to using the site init for adding to sys.path,
importing bazel_tools.tools.python.runfiles stopped working. This is because the
runfiles root was no longer being added to sys.path.

This is somewhat WAI because:

1. Always adding the runfiles root to sys.path is a deprecated legacy behavior because
   it can interfere with Python imports (a repo name can mask a legitimate import).
2. Under bzlmod, repo directory names aren't importable Python names, so having the
   runfiles root on sys.path doesn't do much.

An exception to (2) is bazel_tools: this is special cased to use the directory
name bazel_tools in runfiles. This is where the legacy runfiles library for
Python is.

In any case, forgetting the runfiles root on sys.path for the system_python bootstrap
was an oversight, as the intention was to be a more no-op refactoring.

Fixes #3422

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where Python executables using the system_python bootstrap could no longer import modules from bazel_tools.tools.python.runfiles. The fix involves explicitly re-adding the runfiles root to the Python sys.path during the bootstrap process, but only for system_python implementations, to ensure bazel_tools modules are discoverable. A new test has been introduced to validate this critical import functionality.

Highlights

• Restored bazel_tools importability: The primary change re-enables the import of bazel_tools.tools.python.runfiles for system_python bootstraps, which was broken after a previous refactoring.
• Conditional sys.path modification: The runfiles root is now conditionally added to sys.path only when system_python bootstrap is used, addressing a specific need without reverting to a deprecated global behavior.
• New test for bazel_tools imports: A dedicated test case, bazel_tools_importable_system_python_test, has been added to ensure the correct import behavior of bazel_tools modules under the system_python bootstrap.

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[gemini-code-assist]

Code Review

This pull request addresses an issue where bazel_tools.tools.python.runfiles could not be imported when using the system_python bootstrap implementation, due to the runfiles root not being on sys.path. The fix correctly adds the runfiles root to sys.path conditionally for this bootstrap method. The changes are logical and include a new test to verify the fix. My feedback focuses on improving code clarity and maintainability in the new test file and a related template file.