Build: Use a marker file to indicate forbidden patterns has run

This makes forbidden patterns a little smarter, so it does not need to
run on every build. It works because the marker file timestamp will be
compared against the source files (which are the inputs to forbidden
patterns).

closes #14788

buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/ForbiddenPatternsTask.groovy

@@ -21,6 +21,7 @@ package org.elasticsearch.gradle.precommit
 import org.gradle.api.DefaultTask
 import org.gradle.api.file.FileCollection
 import org.gradle.api.tasks.InputFiles
+import org.gradle.api.tasks.OutputFile
 import org.gradle.api.tasks.OutputFiles
 import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.TaskAction
@@ -36,6 +37,9 @@ class ForbiddenPatternsTask extends DefaultTask {
     Map<String,String> patterns = new LinkedHashMap<>()
     PatternFilterable filesFilter = new PatternSet()
 
+    @OutputFile
+    File outputMarker = new File(project.buildDir, "markers/forbiddenPatterns")
+
     ForbiddenPatternsTask() {
         // we always include all source files, and exclude what should not be checked
         filesFilter.include('**')
@@ -94,6 +98,7 @@ class ForbiddenPatternsTask extends DefaultTask {
         if (failures.isEmpty() == false) {
             throw new IllegalArgumentException('Found invalid patterns:\n' + failures.join('\n'))
         }
+        outputMarker.setText('done', 'UTF-8')
     }
 
     // iterate through patterns to find the right ones for nice error messages