Fix installing updates which need escalated privileges

[joshaber]

⚠️ Dependent on #206 ⚠️

Fixes #157
Fixes #131

There were two problems:

1. We checked if we could write to the app bundle itself and escalated, but we didn't check if we could write to its parent directory to replace the bundle.
2. If we escalated, our user-scoped paths wouldn't be valid since ShipIt would run as root.

I still need to find a way to test this, even if just by hand, to verify the above issues really are fixed.

[joshaber]

I've manually tested the following scenarios:

1. User-owned app bundle and parent directory. Succeeds 👍
2. Root-owned app bundle and user-owned parent directory. Escalates and succeeds 👍
3. Root-owner parent directory. Escalates and succeeds 👍

[hhff]

Hi @joshaber - a big big thankyou for tackling this! Really great stuff 👍

[devtobo]

Is it fixed in Electron (not sure if they or I need to do something to have this fix for my users)?

[joshaber]

No, Electron hasn't yet released an update which includes this fix.

[Roverclover]

Y