Add a new environment variable that disables apk's caching feature for
space constrained environments such as the bpo image build jobs.
Currently we have a workaround in place in bpo: the apk cache is moved
to a tmpfs. But this is fragile and just disabling the apk cache is a
more elegant solution.
I've decided to make this an env var instead of a full pmbootstrap
option since this option is not relevant for normal users, only for CI
jobs in space constrained environments. Also we already have another
`PMB_APK_` env var.
Related: bpo issue 136
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2553
