Run tests also with latest release

[chrysn]

As (or while, but changing that would be difficult) backports are tested with the current image (and not with the one tagged at release time), the CI should stay able to build the latest release. In particular, this broke around Rust when the latest C2Rust was added.

To ensure that, tests of whether RIOT can still be built should not only be executed with the current master but also with the latest release.

The only caveat I see is that this may slow down CI updates -- but breaking things here should be rare, I just want to ensure it doesn't happen accidentally (as it did recently). If a change is necessary and can't be delayed until the next release (which is rather quick at 3 months pace anyway), there is still the option to backport what made master work early and then update CI to use it.

(This is primarily a reminder issue for myself.)