[RFC] .github/workflows/build.yaml: add armv6l, armv7l-musl to build matrix

[classabbyamp]

Testing the changes

• I tested the changes in this PR: NO (but it's a trivial change)

This has been how it's done since at least the switch to github CI (#26095), and how it was done in the travis CI era:

void-packages/.travis.yml

Lines 15 to 21 in 71a25f3

	- ARCH=x86_64 BOOTSTRAP=x86_64
	- ARCH=i686 BOOTSTRAP=i686
	- ARCH=aarch64 BOOTSTRAP=x86_64
	- ARCH=armv7l BOOTSTRAP=x86_64
	- ARCH=x86_64-musl BOOTSTRAP=x86_64-musl
	- ARCH=armv6l-musl BOOTSTRAP=x86_64-musl
	- ARCH=aarch64-musl BOOTSTRAP=x86_64-musl

Should it be changed? Is there a reason why CI isn't currently run on armv6l and armv7l-musl?

[Chocimier]

I think this won't catch any new problems, at cost of lowering throughput of CI.

[classabbyamp]

reasonable enough.