github-ci: build rust doc on stable and 1.34.2

Nothing is done with the rustdoc, its just build to make
sure it builds with our supported versions of Rust

.github/workflows/builds.yml

@@ -179,6 +179,9 @@ jobs:
         env:
           DISTCHECK_CONFIGURE_FLAGS: "--enable-unittests --enable-debug --enable-lua --enable-geoip --enable-profiling --enable-profiling-locks"
       - run: test -e doc/userguide/suricata.1
+      - name: Building Rust documentation
+        run: make doc
+        working-directory: rust
       - name: Preparing distribution
         run: |
           mkdir dist
@@ -276,6 +279,9 @@ jobs:
       - run: make -j2
       - run: make install
       - run: make install-conf
+      - name: Building Rust documentation
+        run: make doc
+        working-directory: rust
 
   fedora-31:
     name: Fedora 31