Add a manually triggerable Github Actions job for Cmdstan RC and release testing

[rok-cesnovar]

Add a manual job that someone with write permissions can start.

Argument: tarball URL

Runs all unit tests with the R release version (no need for R-devel/oldrel) on all OS.

Would serve as a test if cmdstanr works with RCs and as a check for Cmdstan release tarballs - stan-dev/cmdstan#810

[jgabry]

Good idea