Reactivate caching of R packages

[khaeru]

Closes #417.

How to review

View CI logs and ensure that R packages are used from cache and not reinstalled.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes only
• Update release notes.

[codecov]

Codecov Report

Merging #418 (a4df864) into main (1bfdbdd) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #418     +/-   ##
=======================================
- Coverage   97.9%   97.9%   -0.1%     
=======================================
  Files         39      39             
  Lines       4238    4237      -1     
=======================================
- Hits        4150    4149      -1     
  Misses        88      88             

Impacted Files	Coverage Δ
ixmp/tests/test_testing.py	100.0% <100.0%> (ø)

[khaeru]

Compared to the last jobs for #416, times are reduced:

• macos-latest: from 11–12 to 8–11 minutes.
• ubuntu-latest: from 10–12 to 3–4 minutes.
• windows-latest: from 24–30 to 9 minutes