feature: add armv7 prebuilds

[tkurki]

Use qemu and --platform linux/arm/v7 to cross build
binaries for 32 bit arm v7.

Based on #714, see also #601.

Untested, but a slimmed down version seems to build ok.

[m4heshd]

This is a much needed feature and a good approach but there seem to be some issues.

1. This build will fail because you're missing a necessary quotation mark and a new line at the end of the new job
2. It's unnecessary to add a completely new job for this specific build when you can simply use a build matrix

This is implemented in a much cleaner way in #758 since that was already been done for a fork.

[tkurki]

Oooh so much better - all hail #758 !