Allow source installation again

[lorenzwalthert]

Is your feature request related to a problem? Please describe.

Currently, only binary packages from PPM are supported for additional_dependencies. I would like to laxen the requirements upstream to allow attempting source installations. This was previously disabled, since in case the CRAN mirror has a younger version of the R package you want to depend on than your .pre-commit-config.yaml, you will try to install a source package, as only the up-to-date binary is kept in PPM. Since we use frozen snapshots, we will still be able to install a not-up-to-date binary, since for that URL, there won't ever be another one.
Describe the solution you'd like

With the frozen URLs, I think we can allow source installations again, which should only affect packages that don't have a binary version: They now fail to install, in the future, a source installation should be attempted. In managed environments like pre-commit.ci, this might not work, but at least it will work locally and in CI environments the user controls.

Describe alternatives you've considered

Not use packages as dependencies that don't have a binary on PPM for all required operating systems. Request a binary.