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: build.postmarketos.org#136 (closed)

mentioned in issue build.postmarketos.org#136 (closed)

mentioned in commit 4bf3f11b

added 50 commits

• 17dcb535...dcc4137e - 49 commits from branch master
• 4bf3f11b - pmb.helpers.apk: support PMB_APK_NO_CACHE

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(wrong button, did not mean to approve my own MR )

thanks for the reviews, merged without maintainer approval since this has been in the queue for 4 weeks, is fairly simple and is needed to unblock bpo: build.postmarketos.org#155 (closed)

merged