Use filepath.Join when combining filesystem components

[kommendorkapten]

Partly fixes #605

The example-root-signing test should now work on Windows.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 70.51%. Comparing base (171b2e4) to head (208041c).
Report is 25 commits behind head on master.

Files	Patch %	Lines
metadata/updater/updater.go	0.00%	5 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
- Coverage   70.54%   70.51%   -0.04%     
==========================================
  Files          10       10              
  Lines        2122     2123       +1     
==========================================
  Hits         1497     1497              
- Misses        504      505       +1     
  Partials      121      121              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kommendorkapten]

tuf client cli test also works on Windows now and some other, only the multi-repo test that is failing now 🎉

[kipz]

LGTM FWIW

[kommendorkapten]

I would recommend merging this, then I'll look at the multi repo test in a new PR.

[rdimitrov]

🚀