Pre-fetch polyglot snapshot

There's no way to configure extensions.xml to use a snapshot repo,
other than the global settings.xml, so we prefetch it manually.

.github/workflows/ci.yml

@@ -29,6 +29,8 @@ jobs:
           distribution: 'zulu'
           java-version: ${{ matrix.java-version }}
           cache: 'maven'
+      - name: prefetch-polyglot
+        run: "cd .. ; mvn dependency:get -Dartifact=io.takari.polyglot:polyglot-ruby:0.4.11-SNAPSHOT:get -DremoteRepositories=central::default::https://repo.maven.apache.org/maven2,snapshots::::https://oss.sonatype.org/content/repositories/snapshots"
       - name: bootstrap
         run: mvn -Pbootstrap clean package
       - name: bundle install