Do not install python2.7-minimal in the linux-aarch64 firefox container

The first nightly build of the browsers for linux-aarch64 failed. This happened because some native arm64 library dependency of the firefox container (have not identified which, doesn't matter) pulls in the python2.7-minimal:arm64 package, which in its postinst script tries to execute the arm64 python binary. Of course, this fails on a x86_64 build machine. The reason why I never caught this bug is binfmt_misc was enabled on my x86_64 build machine, so my machine was very happy to execute arm64 python using qemu transparently. I could reproduce it by disabling binfmt_misc.

Since python2.7-minimal:arm64 is pulled in just as a recommended package, since it is completely unused in the build, and since the packaging bug (it is one, probably) that pulls in and executes arm64 python affects every single Debian distribution on which python2.7-minimal still exists, the easiest way to fix this is not to install recommended packages in the firefox container for linux-aarch64. For the future, if something similar happens but there's no similarly easy fix, the second easiest solution would be to build and install a dummy (empty) python2.7-minimal:arm64 package.

P.S.: I checked that once this is fixed the build succeeds with binfmt_misc disabled, and produces working binaries.