Fix saving package cache on Windows & older R

[gaborcsardi]

Closes #940.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.61%. Comparing base (9073210) to head (ee73b7e).
Report is 1 commits behind head on v2-branch.

Additional details and impacted files

@@            Coverage Diff             @@
##           v2-branch     #944   +/-   ##
==========================================
  Coverage      84.61%   84.61%           
==========================================
  Files              3        3           
  Lines             13       13           
==========================================
  Hits              11       11           
  Misses             2        2           

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[gaborcsardi]

This is now fixed.

Cached saved:
https://github.com/r-lib/actions/actions/runs/11755531357/job/32750683362?pr=944#step:12:5
and also restored:
https://github.com/r-lib/actions/actions/runs/11755567999/job/32750755693?pr=944#step:5:2091

Newer R as well:
https://github.com/r-lib/actions/actions/runs/11755531357/job/32750683237?pr=944#step:12:5
https://github.com/r-lib/actions/actions/runs/11755567999/job/32750755529?pr=944#step:5:2175

[jeroen]

Awesome thanks!

[gaborcsardi]

@jeroen: Thank you!

[github-actions]

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