Draft: testing/dendrite: build without network access

So checksums will fail as I have no permissions to upload the vendored deps somewhere, but basically these changes allow building a Go program (Dendrite in this case) without network access.

Basically snapshot the deps and upload them to the Alpine sources mirroring service. Then use that in the APKBUILD, unpack it and set the GOFLAG -mod to the unpackaged directory, and done.

I'm unsure how this would allow anyone to upgrade the package however, as of course almost no one has write access to the sources mirror.