The bootstrap installer's build.rs unconditionally baked a commit pin via
`git rev-parse HEAD`, forcing every dev build to clone an exact SHA at
install time. That SHA had to be pushed to origin or the fresh-box clone
would fail.
Make the commit pin opt-in: by default build.rs bakes ONLY the detected
branch, so the installer follows that branch's HEAD at install time. Set
HERMES_BUILD_PIN_COMMIT (SHA, tag, or branch name) to bake an immutable
commit pin for reproducible/release builds; it is resolved to a SHA via
`git rev-parse --verify <ref>^{commit}` and fails loud on an unresolvable
ref. Runtime resolution already supported branch-only pins, so no changes
needed in bootstrap.rs / install_script.rs / install.ps1.
1d9aacbd00