[QOL] Adds occa::env::setOccaCacheDir

[dmed256]

Description

Adds a programmatic way of setting the OCCA_CACHE_DIR

occa::env::setOccaCacheDir(<path>)

Solves #252

[codecov]

Codecov Report

Merging #420 (73c7618) into main (26c1378) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #420      +/-   ##
==========================================
- Coverage   76.31%   76.29%   -0.02%     
==========================================
  Files         241      241              
  Lines       18816    18817       +1     
==========================================
- Hits        14359    14356       -3     
- Misses       4457     4461       +4     

Impacted Files	Coverage Δ
include/occa/tools/env.hpp	80.00% <ø> (ø)
src/tools/env.cpp	84.61% <0.00%> (-2.89%)	⬇️
src/lang/specialMacros.cpp	59.48% <0.00%> (-0.87%)	⬇️
src/tools/sys.cpp	65.22% <0.00%> (-0.17%)	⬇️