The following shell script mounts the Pico and programs the specified firmware file. To be able to mount the Pico it needs the following /etc/fstab entry, the directory /rpi-pico for the mountpoint and the UDEV rules in /etc/udev/rules.d/. Run udevadm control --reload-rules && udevadm trigger after adding the rules file. (these things have to be done as root)
/dev/rpi-pico /rpi-pico vfat user,noauto