Add Fedora 39 builder

[SeanTAllen]

The builder will be used to build nightly and release versions of ponyc targetting Fedora 39.

No release notes are being added at this time, nor is RELEASE_PROCESS.md being updated as until a nightly runs, we can't be sure this will fully work.

The docker image created for the builder can almost certainly be trimmed down in size, but I am not a Fedora/DNF expert. I am looking to the requester of Fedora 39 support to assist with trimming the image down.

[13r0ck]

Oh wow, you got around to this already. Thank you!

[SeanTAllen]

@13r0ck See the Zulip conversation I started for us. I'd appreciate any tips on trimming the image size down via dropping dnf caches or whatever else. Also, if Fedora changes the versions of dependencies by major versions during a Fedora version (like if the clang version might change from say 17 to 18 on a rebuild) then that would be good to address as well.

So... locking versions (if needed because major jumps might happen) and trimming image size down would both be PRs that would be happily accepted (separate PRs please, if needed ).

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The first nightly will run tonight, if everything works, I'll give it a test run in a Fedora 39 container and would appreciate you downloading manually from cloudsmith and trying out as well.

The next scheduled ponyc release is at the end of the month, so once Ponyup is updated, it would be "all nightlies" until the next release when you can also get release builds.