use path/filepath to join the cache path.

[schlubbi]

This will use the path separator that's native to the OS. Partially addressing #1
There seem to be more errors related to the caching.

[codecov-io]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #2   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          16       16           
  Lines         288      288           
=======================================
  Hits          284      284           
  Misses          2        2           
  Partials        2        2

Impacted Files	Coverage Δ
cache/cache.go	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ab0dc8d...f312089. Read the comment docs.

[kirillrogovoy]

@schlubbi, thank you very much for finding time to look into this issue!

I've also found another place where I should've been used filepath.Join as well and fixed that.

I'm also attaching godoc links here for everyone interested:

• https://golang.org/pkg/path/#Join
• https://golang.org/pkg/path/filepath/#Join