add channel arguments to snaps in the action

[ca-scribner]

resolves #27

[ca-scribner]

I see the other channel args were left as default_value=null and then use if statements. I ended up using default=latest/stable so all calls could be snap install ... --channel=${this_channel} to avoid a few extra lines, but I can change it to match the current format if that's preferred.

[johnsca]

In addition to the comment below, you also need to run npm run build. There's a pre-commit hook in the repo under .github/hooks/pre-commit that I recommend you symlink into your .git/hooks/ directory to catch that.

Also, the PR probably doesn't need to be a draft anymore.