CI workflows for sanitizers

[nealrichardson]

We could add a GHA workflow that only runs on changes to /src, and have it test the package using one or more of the R flavors in https://github.com/wch/r-debug. Rsan would have been useful for the most recent issue, but looks like valgrind has been relevant in the past.

[wch]

I don’t know for sure, but it might be better to use the rhub SAN image because (I suspect) there’s more attention on keeping it up to date, and also because the wch1/r-debug image is very large, because it has more R builds than are needed for just SAN checks.