[R] Add verify-rc-source-r job

[assignUser]

Describe the enhancement requested

The new bootstrap.R (#39147) makes it much easier to add an R source verification to the CI/release verification.

To make that viable we have to modify a few things:

• allow nixlibs.R and update-checksums.R to pull from rc dir on jfrog and nightlies.a.o
• add checksums to the r nightly builds
• add R to the verification script
  • Should we just run devtools::build() or also devtools::check_built() or should that be arrows binary verification?

Component(s)

R