Installer should set VOLK_PREFIX and GR_PREFIX

[guruofquality]

The snap support branches have been merged, but only into master. We can add prefix setting now, but it wont take effect until another release.

• Reference: https://github.com/gnuradio/volk/pull/100
• https://github.com/gnuradio/gnuradio/pull/1077

[guruofquality]

Was done: https://github.com/pothosware/PothosSDR/commit/d3c67280717463c1832e40c4ea9ec54d885ea80b