Ensure frontend cache dir does not grow unbounded

[nicksnyder]

The frontend appears to grow unbounded on the shared SSH cache. See https://github.com/sourcegraph/sourcegraph/issues/627#issuecomment-465143874

Here is a similar fix made to lang-go:
sourcegraph/go-langserver#359

This general problem feels like it should be owned by the core services team @keegancsmith @tsenart , unless you have additional information that would indicate there is a better owner. Assigning to you to triage and prioritize.

[keegancsmith]

Related to https://github.com/sourcegraph/sourcegraph/issues/627

[keegancsmith]

#1995 would solve this by moving the cache out of the frontend. Honestly I think this is the correct solution to the problem, so gonna close this out.

Duplicate of #1995