CI: Run a single job on Julia nightly (but skip the incremental=false tests on Julia 1.12)

[DilumAluthge]

1. We intentionally put nightly CI into a separate YAML file (ci.nightly.yml versus the main ci.yml file). This allows us to enable and disable the nightly CI workflow from the GitHub web UI, without having any impact on the main CI.
2. We intentionally only run one CI job on Julia nightly, in order to conserve CI resources.
3. This CI only runs on Julia nightly. It doesn't run on Julia pre-releases. For CI on Julia pre-releases, see the main CI workflow file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.56%. Comparing base (a58c9c1) to head (4598c27).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #982   +/-   ##
=======================================
  Coverage   84.56%   84.56%           
=======================================
  Files           3        3           
  Lines         823      823           
=======================================
  Hits          696      696           
  Misses        127      127           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[DilumAluthge]

CI is failing on Julia nightly, which is obviously not good, but at least we have a CI job now. If the red ❌ gets too distracting, we can easily disable/enable the "Nightly CI" workflow from the GitHub web UI (this page: https://github.com/JuliaLang/PackageCompiler.jl/actions/workflows/ci.nightly.yml) without affecting main CI.