Switch from pass to rbw for retrieving the c-base mail account password.

Allows HedgeDoc editors to maintain focus when clicked, fixing the issue where text input was interrupted after 1 second due to Tridactyl blocking autofocus attempts.

Using the stable system path instead of a hardcoded store path means
BROWSER automatically points to the current system's firefox after rebuild.

Ensures new shells get the latest environment variables even when spawned
from a long-running sway session that was started before nixos-rebuild.