tests/docker: fix debian-all-test-cross

tests/docker: fix debian-all-test-cross

It turns out you can't easily expand an ENV var across multiple steps
in a dockerfile. This meant we silently dropped the architectures we
should have even on amd64 hosts. As the updated AVAILABLE_COMPILERS is
only needed for the following apt install line just merge them.

Fixes: 6da616bb170 (tests/docker: handle host-arch selection for all-test-cross)
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250804104308.250949-1-alex.bennee@linaro.org>