1. 1. Summary

Add back Windows support for the Copybara sync tooling and unit tests, so release
managers can run `bazel run //buildscripts/copybara:generate_evergreen --config=local`
and the Copybara unit tests from Windows.

This restores what SERVER-132895 restricted to Linux and fixes the Windows issues
that motivated it:

• Remove `target_compatible_with = ["@platforms//os:linux"]` from the
  `generate_evergreen_check` (`buildscripts/copybara/BUILD.bazel`) and
  `test_sync_repo_with_copybara` (`buildscripts/tests/BUILD.bazel`) Bazel targets.
• Remove the Linux-only `skipUnless` from the `CopybaraTestCase` base class in
  `buildscripts/tests/test_sync_repo_with_copybara.py`.
• Fix cmd.exe-incompatible shell commands in the unit tests: multi-line `git commit -m`
  messages become separate `-m` flags (cmd.exe splits commands on newlines), and host
  paths in real git commands are quoted with `sync_repo_with_copybara.shell_quote()`
  so Windows temp directories with spaces work.
• Write all generated Copybara files with `newline="\n"` so Windows runs produce LF
  line endings instead of CRLF diffs: `generate_evergreen.py` writes
  `etc/evergreen_yml_components/copybara/copybara_gen.yml`, `path_rules.py` writes
  `copybara_path_rules.bara.sky`, and `sync_repo_with_copybara.py` writes the fetched
  config bundle, assembled and rewritten `copy.bara.sky`, and synthetic fragments.
• Document Windows support and the MSVC toolchain prerequisite in
  `buildscripts/copybara/README.md`.

The Copybara unit tests continue to run in Evergreen only on the Linux copybara sync
variants (`ubuntu2204-small`); they are not wired into any Windows CI build variant.

1. 1. Testing

• `python -m unittest buildscripts.tests.test_sync_repo_with_copybara
  buildscripts.copybara.test_forbidden_text_check` pass locally on Linux.
• `bazel test //buildscripts/copybara:generate_evergreen_check
  //buildscripts/tests:test_sync_repo_with_copybara` pass locally on Linux.
• `python buildscripts/copybara/generate_evergreen.py --check` passes.