Bug 40013: Add option do overwrite timestamp in extension version

build.gradle

@@ -136,7 +136,13 @@ subprojects {
             rename { 'manifest.json' }
             into extDir
 
-            def values = ['version': rootProject.ext.config.componentsVersion + "." + new Date().format('MMddHHmmss')]
+            def systemEnvBuildDate = System.getenv('MOZ_BUILD_DATE')
+            // MOZ_BUILD_DATE is in the YYYYMMDDHHMMSS format. Thus, we only use a
+            // substring of it if it is available.
+            def values = ['version': rootProject.ext.config.componentsVersion + "." +
+                          (systemEnvBuildDate != null ?
+                           systemEnvBuildDate.substring(4) :
+                           new Date().format('MMddHHmmss'))]
             inputs.properties(values)
             expand(values)
         }