This commit passes the `--release-channel=nightly` flag to all images which have
the `DEPLOY` flag set. This means that we'll name artifacts and the compiler
appropriately.

This reworks a bit how arguments are passed, but for now doesn't change what's
already being passed. Eventually we'll want to avoid enabling debug assertions
and llvm assertions for *all* releases, but I figure we can tackle that a little
bit more down the road.