CI: emit the compiler's version in the build reports

Some occasional builds fail only on a specific platform and being able
to figure the exact compiler version used there is crucial. It's not
easy to guess from the rest of the output, so let's add it before the
platform-specific defines, which suit the same needs.

.github/workflows/vtest.yml

@@ -100,6 +100,9 @@ jobs:
       run: make -C addons/wurfl/dummy
     - name: Compile HAProxy with ${{ matrix.CC }}
       run: |
+        echo "::group::Show compiler's version"
+        echo | ${{ matrix.CC }} -v
+        echo "::endgroup::"
         echo "::group::Show platform specific defines"
         echo | ${{ matrix.CC }} -dM -xc -E -
         echo "::endgroup::"