Decouple touchstone dependencies from benchmarkee

[assignUser]

With our use of a fixed RSPM it might cause issues if people use a date where some of {touchstone]'s dependencies where below min version (or we want to use some new package).

With the changes in https://github.com/r-lib/actions/issues/486#event-5961954349 we could split touchstone and benchmarkee install into two step and use up-to-date dependencies for touchstone.

[lorenzwalthert]

Sounds good.

[assignUser]

Actually this will only overwrite the dependencies... we would need some thing like a private library similar to what {pak} does. I'll look into how complex that is and if we can do something similar.