i3lock-fancy: use absolute paths (#17569)

When it's used from command line or as a i3 shortcut, i3lock-fancy works
without any issues. If you try to pass it as an argument to xss-lock or
xautolock running as a systemd user service, getopt and fc-match are not in
PATH and locking doesn't work as expected. This patch replaces non absolute
paths with stable ones and should fix the issue.