Properly do the network-liveness testing for khan-linter updating.

I finally figured out why the `git ls-remote` was timing out so often!
I had intended for it to run in the khan-linter repo, which we know is
rather small.  But it was actually running it whatever directory you
happened to be in (which not be in inside a git repo at all!)  Oops.

Issue: none

Auditors: benkraft

Test plan:
Ran
   ka-lint app.yaml
in webapp/ with success.  Before this change I got a timeout error
when I did that.

runlint.py

@@ -552,6 +552,7 @@ def _maybe_pull():
                 # needs to contact the remote.  If you know of a
                 # faster one, feel free to sub it in!
                 ['git', 'ls-remote', 'origin', 'master'],
+                cwd=os.path.dirname(__file__),
                 stdout=devnull, stderr=devnull)
             if rc != 0:
                 raise RuntimeError('git ls-remote returned %s' % rc)