ci, macos: Install pkgconf Homebrew's package

The updated GHA image `20241125.556` is now fully deployed, enabling the
installation of `pkgconf` as documented in `doc/build-osx.md`.

.github/workflows/ci.yml

@@ -120,8 +120,7 @@ jobs:
         run: |
           # A workaround for "The `brew link` step did not complete successfully" error.
           brew install --quiet python@3 || brew link --overwrite python@3
-          # Update to install pkgconf, once the GHA image has been updated.
-          brew install --quiet coreutils ninja gnu-getopt ccache boost libevent zeromq qt@5 qrencode
+          brew install --quiet coreutils ninja pkgconf gnu-getopt ccache boost libevent zeromq qt@5 qrencode
 
       - name: Set Ccache directory
         run: echo "CCACHE_DIR=${RUNNER_TEMP}/ccache_dir" >> "$GITHUB_ENV"