Avoid race conditions when asking the user to import a build.

Previously, a race condiition could cause Metals to prompt the user
twice to import build changes. The duplicate prompt could cause some
test suites to become flaky. Now, we synchronize on the method that
prompts the user to ensure that we finish writing the digest
notification status in the H2 database before sending the next prompt.

metals/src/main/scala/scala/meta/internal/metals/BloopInstall.scala

@@ -135,10 +135,14 @@ final class BloopInstall(
     }
   }
 
+  // NOTE(olafur) there's a chance that we get two build change notifications in
+  // a very short period due to duplicate `didSave` and file watching
+  // notificiations. This method is synchronized to prevent asking the user
+  // twice whether to import the build.
   def runIfApproved(
       buildTool: BuildTool,
       digest: String
-  ): Future[BloopInstallResult] = {
+  ): Future[BloopInstallResult] = synchronized {
     oldInstallResult(digest) match {
       case Some(result) =>
         scribe.info(s"skipping build import with status '${result.name}'")