ci: Occasional failure to build stdlib pkgimages

[Keno]

Occasionally our CI workers seem to be failing to finish the build of the stdlib pkg images, e.g. here:

https://buildkite.com/julialang/julia-master/builds/30327#018bf1bc-f65b-4903-9bdb-682013cdc24f

I feel like I've mostly seen this on the macOS workers, but it's very possible that's just a scheduling difference.

[vtjnash]

I thought you had previously identified there are bugs in the REPL precompile script (which looks like where it got stuck) that cause it to get stuck sometimes

[Keno]

Well, I thought I fixed those, but this one appears to have gotten stuck in

make[1]: *** [stdlib/MozillaCACerts_jll.release.image] Terminated: 15