sync: fix os.environ logic errors

This is a dict to index, not a function to call.

Change-Id: I0117eeaaa8b2ef4762ab6f0d22f9ffdaee961f52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258132
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

subcmds/sync.py

@@ -729,12 +729,12 @@ later is required to fix a server side protocol bug.
           branch = branch[len(R_HEADS):]
 
         if 'SYNC_TARGET' in os.environ:
-          target = os.environ('SYNC_TARGET')
+          target = os.environ['SYNC_TARGET']
           [success, manifest_str] = server.GetApprovedManifest(branch, target)
         elif ('TARGET_PRODUCT' in os.environ and
               'TARGET_BUILD_VARIANT' in os.environ):
-          target = '%s-%s' % (os.environ('TARGET_PRODUCT'),
-                              os.environ('TARGET_BUILD_VARIANT'))
+          target = '%s-%s' % (os.environ['TARGET_PRODUCT'],
+                              os.environ['TARGET_BUILD_VARIANT'])
           [success, manifest_str] = server.GetApprovedManifest(branch, target)
         else:
           [success, manifest_str] = server.GetApprovedManifest(branch)