github: combine nightly jobs with default jobs using matrix

[phlogistonjohn]

At this point the job definitions for build-nightly- and test-nightly- were nearly the same as the non-nightly counterparts. Combine the job definitions via a new matrix parameter.

[phlogistonjohn]

I updated the matrix and the matching rules in mergify config. However, this will need to have manual changes made to the github ui too, like the last set of changes that added the matrix.