Windows build fixes for Bazel ≥ 9.0.1

[walter-zeromatter]

• bootstrap_process_wrapper.cc: rewrite @response-files to convert the rustc --sysroot= value to its 8.3 short form via GetShortPathNameA before spawning rustc. The MSVC link.exe shipped with VS 2022 BuildTools lacks longPathAware, so the resolved <sysroot>/lib/rustlib/.../libstd-*.rlib paths overflow MAX_PATH (260) and fail with LNK1181. Also restrict get_output_base to follow external/ only when it's a symlink (Bazel ≥ 9.0.1 made it a real directory on Windows).

• options.rs: same external-is-real-dir fix for the regular process_wrapper. Falls back to deriving output_base from current_dir's grandparent when external/ is not a symlink.

• incremental_cache.rs: find_output_base prefers an ancestor whose basename is not execroot. Bazel writes DO_NOT_BUILD_HERE in both the real output_base and execroot/ on Windows ≥ 9.0.1, and picking the closer one yields a doubled …/execroot/execroot/_main/… path.

• main.rs: skip *.rcgu.{o,bc,bc.z} codegen intermediates when building the unified -Ldependency dir, and tolerate NotFound on the copy fallback. Pipelined sibling actions can garbage-collect those temporaries between read_dir and the hardlink/copy.

• cargo_build_script_runner/bin.rs: track exec_root symlinks for cleanup only when symlink_if_not_exists actually created the symlink (was hitting DirectoryNotEmpty trying to remove_dir real dirs created by runfiles staging, e.g. .github). Also skip .git from execroot symlinks so ring ≤ 0.17.x doesn't enter its "local hacking" mode that hardcodes ./target/tools/windows/nasm/nasm.

• cargo_build_script_runner/cargo_manifest_dir.rs: use symlink_metadata in remove_symlink so dangling directory symlinks (Bazel's local-spawn-runner.* sandboxes cleaned up before this runs) get remove_dir'd instead of failing remove_file with ACCESS_DENIED.