Drop CI for Julia Nightly

[oxinabox]

we need to drop Julia-Nightly from the CI (out of scope for this PR)

Originally posted by @oxinabox in #130 (comment)

allow failures doesn't work on GHA

[sethaxen]

Why do we need to drop it? Nightlies are run in a separate workflow from CI precisely so that failing status on nightlies does not produce a failing status for CI. It will show ❌ on all commits whenever nightly is failing, but that's also true for random codecov issues and the like. At the bottom of each PR or whenever one clicks on the ❌ , nightlies are separated from the rest of the CI.

[oxinabox]

I guess we don't.
I worry it is boy who cried wolf.
One day I will ignore the wrong cross