This reverts commit 8be0992.

We need to provide R_user_dir because pkgcache cannot
import it, and also cannot `::` it because it does not
exist on R 3.6.x or earlier.

The git app needs them.

`prop$show_sysreqs()` prints an empty line if
sysreqs are supported, but there are no system
requirements for the proposed packages.
We get rig of that empty line now.

This should fix some failing snapshots on Linux.

'pak repository:' is not included on the CI.
(Not sure why, though.)

Getting stray build failures about this...

[ci skip]