Reverts two commits:
977cd530c7820b7e432b
These break build from source on Linux for two reasons:
1.) The systemd user service needs to be installed in the `share`
prefix, not the `lib` prefix. This lets it get picked up in `~/.local`
but is also correct for just standard FHS paths.
2.) The `ghostty` path in the systemd user service needs to be absolute.
We should interpolate in the build install prefix to form an absolute
path.