Set up CI testing against Numba `main`

[kkraus14]

Presently numba-cuda CI tests against the last Numba release version. This is not sufficient to catch incompatibilities that may arise between upstream main and numba-cuda, so additional CI setup is needed to test against Numba main.

[leofang]

Q: With #778 in mind, is this issue still relevant?

[kkraus14]

There are still some places where numba types and objects are optionally used if available, so I think this is still somewhat relevant, though unlikely to be prioritized