Use lcov HEAD for now on macOS

Because lcov 2.2 (and, so far, 2.3-beta) has regex issue that Perl
has starte complaining about:

> Unescaped left brace in regex is deprecated here (and
> will be fatal in Perl 5.32), passed through in regex;

The upstream fix (not released yet) is here:
b90b978

.github/workflows/build.yaml

@@ -200,7 +200,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install lcov
-        run: brew install lcov
+        run: brew install lcov --head # \todo Remove when lcov commit b34c844 is released.
+        # \todo See https://github.com/linux-test-project/lcov/commit/b34c84431a97b49e55785286fa05e9a95b873920
       - name: Install Qt
         uses: jurplel/install-qt-action@v4
         with: