pmb.helpers.apk: support PMB_APK_NO_CACHE

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: !2553