Opportunities for making workflows more efficient

[glin]

There are a few places where we could cut out some repeated work and make the workflows more efficient:

• Each R version/distro combination is effectively building the same source package, which could also cause issues if trying to upload with the --replace flag (Consider adding example/documentation for overwriting an existing package #10). You could use a matrix parameter to flag a single R version/distro combination for source package building, but it's tricky with how the matrix is done now with both R version and distro being their own variables. One way to get around that could be to include an additional matrix config (https://docs.github.com/en/actions/using-jobs/using-a-matrix-for-your-jobs#example-expanding-configurations)
• Separate jobs are created for every RSPM endpoint right now, so each source/binary package is being built 3 times even though they're the same package. Some users might want completely identical packages (i.e., same build timestamp) to be uploaded to staging/prod instances as well. We could move the RSPM endpoint/tokens out of the build matrix and into some global list that each job iterates through.