Make "./mach geckolib" use the stable Rust compiler

.gitignore

@@ -6,6 +6,7 @@
 /ports/android/libs
 /ports/android/local.properties
 /ports/android/obj
+/ports/geckolib/target
 /python/_virtualenv
 /python/tidy/servo_tidy.egg-info
 *~

python/servo/build_commands.py

@@ -356,6 +356,7 @@ def build_cef(self, jobs=None, verbose=False, release=False):
                      action='store_true',
                      help='Build in release mode')
     def build_geckolib(self, jobs=None, verbose=False, release=False):
+        self.set_use_stable_rust()
         self.ensure_bootstrapped()
 
         ret = None
@@ -367,11 +368,12 @@ def build_geckolib(self, jobs=None, verbose=False, release=False):
         if release:
             opts += ["--release"]
 
-        build_start = time()
         env = self.build_env()
+        env["CARGO_TARGET_DIR"] = path.join(self.context.topdir, "ports/geckolib/target")
+
+        build_start = time()
         with cd(path.join("ports", "geckolib")):
-            ret = call(["cargo", "build"] + opts,
-                       env=env, verbose=verbose)
+            ret = call(["cargo", "build"] + opts, env=env, verbose=verbose)
         elapsed = time() - build_start
 
         # Generate Desktop Notification if elapsed-time > some threshold value