don't touch `bp2build_workspace_marker` file if it exists

- the generating rule has `restat=true`, so its output need not have a newer timestamp to prevent it from rerunning on subsequent ninja invocations
- its dependents (so far only bootstrap.ninja's "build build.ninja") already have proper dependencies in the depfile, namely
  1. Android.bp.list,
  2. entries in Android.bp.list,
  3. bazel.list and
  4. entries in bazel.list

This change prevents symlink related changes from spuriously retriggering build.ninja

Change-Id: I93f1fea7054dfbfc7c13ece34d2d1f07a81bbe07
Test: manually verifying bazel analysis is triggered only under the right set of CUJs using build/bazel/ci/incremental_build.py
Bug: b/239044236