Upgrade execa back to 8.x

[marcopiraccini]

We had to go back to execa 5.
The reason is that from 6 only ESM is supported in execa, BUT exlectron compiles to JS with CommonJS (actually transforming the import in require`. This would nt be an issue if we use dynamic imprt (like we did) BUT the dynamic import in electron is broken (see: electron/electron#38957) and will be fixed with electron 0.29.

So when electron 0.29 will be available, upgrade execa (and use dymaic import to lad it)

[mcollina]

You can also avoid execa entirely and just use child_process.

[marcopiraccini]

Actually, not solve yet :( alex8088/electron-vite#401