[CI] Switch from debian testing to unstable
[CI] Switch from debian testing to unstable

testing is frozen in preparation for release => no clang-12

darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed