MainWindow: fallback to checking for installer if proc/cmdline fails

[danirabbit]

In #733 we switched methods for checking for proc/cmdline. Add a fallback to the old method of looking for the installer in case that doesn't work for some reason

Also go ahead and select that default session if it exists

Might fix #734 🤷‍♀️

[danirabbit]

I don't see anything in journalctl that would imply that this is failing, but doesn't hurt to have a fallback. It's more likely the issue is that select session bit

[ryonakano]

This didn't worked for me unfortunatelly:

FYI I'm testing with the following steps:

1. Install this branch and the latest Installer main branch
2. Fake the /proc/cmdline with the following command: cat /proc/cmdline | tr -d '\n' > /tmp/cmdline; echo " boot=casper" >> /tmp/cmdline && sudo mount -B /tmp/cmdline /proc/cmdline
3. Logout
4. Don't forget sudo umount /proc/cmdline after testing

[ryonakano]

Closing since #733 is reverted in #736.