respect XDG_CACHE_HOME

[anarcat]

fixes attic#181

[codecov-io]

Current coverage is 83.09%

Merging #247 into master will increase coverage by +0.01% as of 8acf824

@@            master    #247   diff @@
======================================
  Files           31      31       
  Stmts         6698    6714    +16
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit           5565    5579    +14
  Partial          0       0       
- Missed        1133    1135     +2

Review entire Coverage Diff as of 8acf824

---

Uncovered Suggestions

1. +0.63% via borg/fuse.py#124...165
2. +0.51% via borg/xattr.py#199...232
3. +0.43% via borg/xattr.py#166...194
4. See 7 more...

Powered by Codecov. Updated on successful CI builds.