New version of spack seems to break the tool

[toxa81]

I'm getting this error

spack -e ./gcc buildcache create --rebuild-index --allow-root --only=package -m alpscache \
$(spack --color=never -e ./gcc find --format '{name};{/hash}' | grep -v -E '^();' | cut -d ';' -f2)
==> Error: unrecognized arguments: -m

when trying to build https://github.com/eth-cscs/alps-spack-stacks/pull/10

[RMeli]

The -m option was deprecated in 0.20 for removal in 0.21 (see https://github.com/eth-cscs/stackinator/pull/34#discussion_r1094541231). The mirror is now a positional argument.