Fix cache dir location

[dham]

TODO

• There should be a single FIREDRAKE_CACHE_DIR environment variable, and the TSFC and PyOP2 chaches should default subdirectories of it.
• The default value of FIREDRAKE_CACHE_DIR should be $VIRTUAL_ENV/cache

RATIONALE

• Independently setting the locations of the two caches is almost never needed and just confuses users.
• Having different firedrake installs share caches is a potential source of bugs.

[miklos1]

Thought it would be nice to set cache dir in firedrake_configuration.

[dham]

Potentially yes, though this would require a mechanism for users to modify the firedrake_configuration accordingly (currently it just records what they did during install)

[wence-]

We could set it in firedrake parameters, and have an update callback that pushes it out to os.environ?

[dham]

Isn't it the other way around?

[wence-]

I was just thinking about how you inform PyOP2 where the cache directory is.

[dham]

cf #1113

[wence-]

File arrive as part of #1108.

[wence-]

Fixed in #1114.