gr-gsm-master: migrate from archlinux to debian-bullseye-build
Arch Linux ships gnuradio-3.9.4.0, while gr-gsm has not been migrated
to 3.9 yet. Let's use debian:bullseye, which ships gnuradio-3.8.2.0.
Change-Id: I9900addf152fd254283ef340cea16583ee5ac1c5
