Add the environment variable to avoid gem install error in GitHub Actions.

This environment variable is necessary to avoid `gem install` error
on a Windows in GitHub Actions.
See also:
jruby/jruby#7182 (comment)

Author: hiroyuki-sato

embulk-ruby/build.gradle

@@ -74,6 +74,11 @@ task installDependencyGems(type: JavaExec, dependsOn: "gemContents") {
     main = "org.jruby.Main"
     args = ["-rjars/setup", "-S", "gem", "install", "msgpack:1.1.0"]
     environment "GEM_HOME": "${buildDir}/dependencyGems"
+    // This environment variable is necessary to avoid `gem install` error
+    // on a Windows in GitHub Actions.
+    // See also:
+    // https://github.com/jruby/jruby/issues/7182#issuecomment-1109615503
+    environment "DEBUG_RESOLVER": "1"
 }
 
 task rubyTest(type: JavaExec, dependsOn: [