[r/ci] Centralize r-universe stanzas from various R CI YAML files

[johnkerl]

At present we have three R CI YAML files which involve installing a correct/compatible version of TileDB-R:

• .github/workflows/r-ci.yml
• .github/workflows/r-python-interop-testing.yml
• .github/workflows/r-valgrind.yaml

It is possible that we would extract out the common/reused blocks into a helper YAML file.

But also note that via C++ification work we will be entirely eliminating TileDB-SOMA's dependency on TileDB-R and TileDB-Py both, at which point the above-referenced stanzas will disappear entirely.