Use uppercase drive letter for SERVO_CACHE_DIR

Windows apparently doesn't recognize lowercase drive letters.
servo · bors-servo · May 6, 2016 · May 6, 2016 · May 6, 2016 · 52e831cd7d39c060fbeeee41c8e9fdfec82bd4fd
commit 52e831cd7d39c060fbeeee41c8e9fdfec82bd4fd
diff --git a/buildbot/master/files/config/environments.py b/buildbot/master/files/config/environments.py
@@ -41,7 +41,7 @@ def __add__(self, other):
         r'C:\Windows\System32\WindowsPowerShell\v1.0',
         r'C:\Program Files\Amazon\cfn-bootstrap',
     ]),
-    'SERVO_CACHE_DIR': r'c:\msys64\home\Administrator\.servo',
+    'SERVO_CACHE_DIR': r'C:\msys64\home\Administrator\.servo',
 })
 
 build_mac = build_common + Environment({