`main` launch time regression v.s. `0.12.x`

[lihaoyi]

On main,

• time /Users/lihaoyi/Github/mill/out/dist/native/executableRaw.dest/native-executable version takes ~450-500ms
• time ./mill version (using 0.12.7) takes ~200ms

We don't need to fix this urgently, but we should make sure to come back to it before we release 0.13.0