ci: ensure gem-test will work for java

bundler's behavior is different between CRuby and JRuby, where the
JRuby impl (at least in the docker image) sets the env var
BUNDLE_GEMFILE in what I consider to be an inappropriate manner.

in this commit we also avoid an extra `bundle install` by reordering
operations and using `--skip-install`.

concourse/tasks/gem-test/gem-install-and-test.sh

@@ -14,10 +14,14 @@ popd
 
 pushd nokogiri
 
+  export BUNDLE_GEMFILE=$(pwd)/Gemfile
+  bundle -v
+  bundle config
+
+  bundle add nokogiri --skip-install
   bundle install --local || bundle install
-  bundle add nokogiri
   bundle show nokogiri
+
   bundle exec rake test
 
 popd
-