Support custom cache dir

[mrkam2]

In our CI we want to run several clojure tasks for the same project in parallel and they sometimes fail with:

Refreshing classpath
Error building classpath. Can't create directory: /path/to/project/.cpcache

due to a race condition. I'm proposing this change to make the cache configurable.

Thoughts?

[mrkam2]

I believe race condition is happening in https://github.com/clojure/tools.deps/blob/master/src/main/clojure/clojure/tools/deps/util/io.clj#L53-56

When parent doesn't exist, it proceeds to the .mkdir. But if directories already exist by the time .mkdir creates them, it returns false.

Alternatively we could be checking if the directory exists again after .mkdir returned false as in https://github.com/clojure/tools.deps.alpha/pull/3/files#diff-9d69f30572d7d2f8b1b5b4216257178933c0d73ef285ae66bc97ae5acdab59cfR53-R57

[puredanger]

Hi, this project doesn't support pull requests so I'm going to close the PR, but please go to https://ask.clojure.org/index.php/13671/race-condition-in-cpcache-directory-creation to vote for this issue or provide info there. I do believe your assessment as to the source of the problem is correct, probably more likely that we will try to address it in the script than add more config.