build --strict: fix for building two devices in a row

Fix the issue of having the postmarketOS binary repository key deleted from the chroots after "abuild undeps" removes the postmarketos-keys package. This happens if e.g. building two device packages in a row (as they depend on postmarketos-base, which depends on postmarketos-keys in current pmaports.git master).

I've also considered installing the postmarketos-keys next to alpine-base in new chroots (as in !2108 (closed)). But this would introduce a bootstrap problem, since you can't install the postmarketos-keys package unless it already exists in the repository. We'd run into that when building the next release.

Fixes pmaports#1230 (closed), see commit messages for details.