[ci] Build both DEBs and RPMs by default

The build is now fast enough that we can build both faster than we used
to build each individual type.

For context, the fully cached (with `ccache`) build for both takes a
total of about 7 minutes, whereas a non-cached full build of just the
DEBs used to take about 12 minutes. I expect that on average, building
them both when there are source changes (i.e. when only part of the
build is cached) would likely take a total of about 10 minutes.