github-ci: in a dist build, check that --frozen is being used

Verify that ./configure is picking up the vendored Rust sources
when building from a dist archive.

.github/workflows/builds.yml

@@ -148,6 +148,9 @@ jobs:
         run: tar zxvf ../dist/suricata-*.tar.gz --strip-components=1
       - working-directory: suricata
         run: ./configure
+      - working-directory: suricata
+        name: Check that Rust is using --frozen
+        run: grep -q '^FROZEN' rust/Makefile
       - working-directory: suricata
         run: make -j2
       - working-directory: suricata