Fix setting atexit. Fixes trailing download tasks during precompilation

[IanButterworth]

Fixes JuliaLang/Pkg.jl#4004
Fixes JuliaLang/julia#52685

% JULIA_DEPOT_PATH=$(mktemp -d): ./julia --start=no -e 'using Pkg; Pkg.add("MicroMamba");'
  Installing known registries into `/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/tmp.pCuLh8wMW6`
       Added `General` registry to /var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/tmp.pCuLh8wMW6/registries
    Updating registry at `/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/tmp.pCuLh8wMW6/registries/General.toml`
   Resolving package versions...
   Installed micromamba_jll ─ v1.5.8+0
   Installed MicroMamba ───── v0.1.14
   Installed JLLWrappers ──── v1.5.0
   Installed Scratch ──────── v1.2.1
   Installed Preferences ──── v1.4.3
    Updating `/private/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/tmp.pCuLh8wMW6/environments/v1.12/Project.toml`
  [0b3b1443] + MicroMamba v0.1.14
    Updating `/private/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/tmp.pCuLh8wMW6/environments/v1.12/Manifest.toml`
  [692b3bcd] + JLLWrappers v1.5.0
  [0b3b1443] + MicroMamba v0.1.14
  [21216c6a] + Preferences v1.4.3
  [6c6a2e73] + Scratch v1.2.1
  [f8abcde7] + micromamba_jll v1.5.8+0
  [0dad84c5] + ArgTools v1.1.2
  [56f22d72] + Artifacts v1.11.0
  [2a0f44e3] + Base64 v1.11.0
  [ade2ca70] + Dates v1.11.0
  [f43a241f] + Downloads v1.6.0
  [7b1f6079] + FileWatching v1.11.0
  [dc6e5ff7] + JuliaSyntaxHighlighting v1.12.0
  [4af54fe1] + LazyArtifacts v1.11.0
  [b27032c2] + LibCURL v0.6.4
  [76f85450] + LibGit2 v1.11.0
  [8f399da3] + Libdl v1.11.0
  [56ddb016] + Logging v1.11.0
  [d6f4376e] + Markdown v1.11.0
  [ca575930] + NetworkOptions v1.2.0
  [44cfe95a] + Pkg v1.12.0
  [de0858da] + Printf v1.11.0
  [9a3f8284] + Random v1.11.0
  [ea8e919c] + SHA v0.7.0
  [f489334b] + StyledStrings v1.11.0
  [fa267f1f] + TOML v1.0.3
  [a4e569a6] + Tar v1.10.0
  [cf7118a7] + UUIDs v1.11.0
  [4ec0a83e] + Unicode v1.11.0
  [deac9b47] + LibCURL_jll v8.6.0+0
  [e37daf67] + LibGit2_jll v1.8.0+0
  [29816b5a] + LibSSH2_jll v1.11.0+1
  [c8ffd9c3] + MbedTLS_jll v2.28.6+0
  [14a3606d] + MozillaCACerts_jll v2024.3.11
  [83775a58] + Zlib_jll v1.3.1+0
  [8e850ede] + nghttp2_jll v1.60.0+0
  [3f19e933] + p7zip_jll v17.5.0+0
Precompiling all packages...
  5 dependencies successfully precompiled in 6 seconds. 26 already precompiled.
  1 dependency had output during precompilation:
┌ micromamba_jll
│   Downloading artifact: micromamba
└