`tox -e snap` does not work on Jammy Jellyfish

[Jason C. Nucciarone]

The main command to build the microstack snap package - tox -e snap - does not work on Jammy Jellyfish as this series does not have Firefox installed as a deb package but rather a snap package. In turn, firefox-geckodriver is no longer available from the Ubuntu archive, but rather packaged in the snap as well. `apt search firefox-geckodriver` will not return anything.

For future travelers, `apt-get install firefox firefox-geckodriver` in lxd_build.sh should be changed into `snap install firefox`.