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