use absolute path in cache file system backend

[xhdong-umd]

For #65

[codecov-io]

Codecov Report

Merging #73 into master will decrease coverage by 17.82%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #73       +/-   ##
===========================================
- Coverage   76.28%   58.46%   -17.83%     
===========================================
  Files           5        5               
  Lines         194      195        +1     
===========================================
- Hits          148      114       -34     
- Misses         46       81       +35

Impacted Files	Coverage Δ
R/cache_filesystem.R	100% <100%> (ø)	⬆️
R/cache_s3.R	0% <0%> (-100%)	⬇️

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[jimhester]

Can you put this before the conditional please, and add a note to NEWS.md with the fix, issue number and your GitHub username, thanks!

[xhdong-umd]

If you mean to convert before line 30, it will fail when user provided a new folder name that not created yet. I put it after the folder creation specifically to avoid that.

And in the initialization of file system cache it should take current working directory with relative path, after that all the maintenance functions of the backend will use the absolute path internally so user can change working directory without problem.