Bug 1753914 - ./mach environment is failed in gradle task on Windows. r=nalexander

When driver letter is upper case, `commandLine.execute` seems to be failed.
So I would like to change MOZILLABUILD environment variable to lower case.

Differential Revision: https://phabricator.services.mozilla.com/D137976

settings.gradle

@@ -9,9 +9,9 @@ if (System.properties['os.name'].toLowerCase().contains('windows')) {
     // gradle is called before parsing config.status, we cannot use PYTHON
     // value.
     if (System.env.MOZILLABUILD) {
-        def mozillabuild = System.env.MOZILLABUILD
+        def mozillabuild = System.env.MOZILLABUILD.toLowerCase()
         if (mozillabuild) {
-            commandLine.addAll(0, ["${mozillabuild}/python3/python.exe"])
+            commandLine.addAll(0, ["${mozillabuild}/python3/python3.exe"])
         }
     }
 }