fix(build): work with Git-less Homebrew

Historically, Homebrew cloned the homebrew/core tap using Git. Nowadays,
Homebrew might run in a mode where the tap is not cloned from Git. This
causes 'git rev-parse HEAD' in collect-copyright to fail.

Use the tap_git_head data from the INSTALL_RECEIPT.json file instead.
This data is more accurate and it works in Git-less Homebrew.

tools/collect-copyright

@@ -538,13 +538,9 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
         tap = install_receipt["source"]["tap"]
         if tap != "homebrew/core":
             raise Exception(f"Unsupported tap for formula {formula_name}: {tap}")
+        tap_git_commit = install_receipt["source"]["tap_git_head"]
 
         formula_rb_path = pathlib.Path(install_receipt["source"]["path"])
-        # TODO(strager): Get the exact version installed, not the latest
-        # version.
-        tap_git_commit = subprocess.check_output(
-            ["git", "rev-parse", "HEAD"], encoding="utf-8", cwd=formula_rb_path.parent
-        ).rstrip("\n")
         formula_rb_repo_path = self._relative_path_within_git_checkout(formula_rb_path)
 
         return self.InstalledPackage(