build: avoid chmod through path symlinks

[officialasishkumar]

Summary

• skip the automatic chmod/chown pass after paths symlink mutations
• skip symlink entries during recursive directory permission walks
• add tests for symlinks to missing targets, preserving target directory permissions, and recursive walks over broken symlinks

Fixes #1270.
Fixes #1565.

Validation

• go test ./pkg/build -run 'TestMutatePathsSymlinkAllowsMissingTarget|TestMutatePathsSymlinkDoesNotChangeTargetPermissions|TestMutateDirectoryRecursiveSkipsSymlinks' -count=1\n- git diff --check\n\nNote: I also attempted go test ./pkg/build -count=1, but the process stalled in the Go link step on this shared machine and was terminated to avoid leaving it running.\n