Glean Gradle plugin: Ensure offline mode is offline for pip as well

Previously it was in theory possible that it queries the Pypi index for
packages if it can't find anything in the cache or wheels directory.

gradle-plugin/src/main/groovy/mozilla/telemetry/glean-gradle-plugin/GleanGradlePlugin.groovy

@@ -406,6 +406,7 @@ except:
                     args "pip"
                     args "install"
                     args "glean_parser"
+                    args "--no-index"
                     args "-f"
                     args pythonPackagesDir
                 }