Improve reuse of `/compiled` cache in CI

[IanButterworth]

It would be nice to speed up julia CI startup time

Take GitHub julia CI, which typically

• downloads any registries
• downloads packages
• downloads any artifacts
• builds all packages
• precompiles all packages with test args (--coverage and --check-bounds etc.)
• runs tests

https://github.com/julia-actions/cache provides a way to cache useful parts of the depot between runs.

However two of the above are disabled by default:

• /registries which seems the path is clear to that being enabled now enable registries by default julia-actions/cache#62
• /compiled which means all packages are precompiled each run.

I tried enabling caching of /compiled over at JuliaIO/PNGFiles.jl#66

However on the second run it hits re-precompilation for most of the packages because:

1. pkgimages are disabled in the test environment due to coverage being on by default on julia-runtest
2. We only ship julia with stdlib caches with pkgimages on (default and --check-bounds=yes)

i.e.

 Debug: Rejecting cache file /opt/hostedtoolcache/julia/1.9.2/x64/share/julia/compiled/v1.9/MbedTLS_jll/u5NEn_RmBmg.ji for  [top-level] since the flags are mismatched
│   current session: use_pkgimages = false, debug_level = 1, check_bounds = 1, inline = true, opt_level = 2
│   cache file:      use_pkgimages = true, debug_level = 1, check_bounds = 1, inline = true, opt_level = 2
└ @ Base loading.jl:2730

Two ways to improve this caching would be to

• Enable pkgimages in more CI because coverage being on by default means no pkgimages - Don't use pkgimage for package if any includes fall in tracked path for coverage or alloc tracking #48183 which I stalled on. Help appreciated
• Include pkgimage=no caches for stdlibs include --pkgimage=no caches for stdlibs #50666

### Tasks
- [ ] https://github.com/JuliaLang/julia/issues/50918
- [ ] https://github.com/JuliaLang/julia/pull/49866
- [ ] https://github.com/JuliaLang/julia/pull/50666
- [ ] https://github.com/JuliaLang/julia/pull/52123
- [ ] https://github.com/julia-actions/cache/pull/71
- [ ] https://github.com/julia-actions/julia-runtest/issues/89
- [ ] https://github.com/julia-actions/julia-buildpkg/issues/40
- [ ] https://github.com/julia-actions/setup-julia/pull/196

[IanButterworth]

Also, if both /registries and /compiled become default on, https://github.com/julia-actions/cache becomes quite powerful so https://github.com/julia-actions/julia-runtest could be trained to suggest the cache action if it sees that a depot doesn't exist yet.

[IanButterworth]

Now that #52123 is merged we're able to see the effect of caching /compiled over at julia-actions/cache#71

For an install and load-only CI run, 1.11 goes from 2 minutes to 40s.

[IanButterworth]

v1.9.5 of julia-actions/setup-julia fixes this now for all julia versions, by maintaining the mtime of the julia source files from the archives.

You can see that in this run no platform re-precompiled anything but the checked out package https://github.com/IanButterworth/PowerMonitor.jl/actions/runs/7425482162/job/20207208161

The only remaining things to do is to get more CI to use julia-actions/cache which I'm proposing by getting other actions to notify if caching isn't detected

• Detect if no depot and recommend julia-actions/cache julia-actions/julia-runtest#89
• Detect if no depot and recommend julia-actions/cache julia-actions/julia-buildpkg#40

But this is now just an adoption issue, so I think we can close this.

And for those looking to get these benefits, you just need to specify v1 of setup-julia and cache as shown here https://github.com/julia-actions/cache#usage