[llvm] Use on-disk cache as benchmark scratch directory

[ChrisCummins]

Fixes #465.

[codecov-commenter]

Codecov Report

Merging #510 (65a6da8) into development (87e4f7e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #510      +/-   ##
===============================================
- Coverage        87.47%   87.46%   -0.02%     
===============================================
  Files              113      113              
  Lines             6397     6397              
===============================================
- Hits              5596     5595       -1     
- Misses             801      802       +1     

Impacted Files	Coverage Δ
compiler_gym/envs/llvm/datasets/cbench.py	77.44% <100.00%> (ø)
compiler_gym/service/connection.py	78.71% <0.00%> (-0.68%)	⬇️
...loop_tool/service/loop_tool_compilation_session.py	89.26% <0.00%> (+0.67%)	⬆️

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