No inputs disables snap install

[barrettj12]

If this action is invoked with no channel input, it will not attempt to refresh LXD from snap - so you can use the pre-installed LXD version.

Also added some more GitHub tests.

[barrettj12]

On second thought, maybe channel: '' (or unset) should be reserved for the default channel (IINM this can be different from latest/stable?), and we should have an explicit disable-snap-install input.

[taurus-forever]

I personally prefer the explicit configuration options (e.g. disable-snap-install) and not a magic values.
On a long run it always makes easier to deprecate the option, etc.
Deprecating the value is a way harder.

Also, essentially user might expect identical behavior for:

    - name: Setup LXD
      uses: canonical/setup-lxd@[version]
      with:
        channel: ''

and

    - name: Setup LXD
      uses: canonical/setup-lxd@[version]

While they are dramatically different. It can lead to bad surprises for the end-users.
You are the author, the PR will work for me, I will obey. LGTM.

PR#4 is updated, I close it in favor of this PR.

[barrettj12]

Yeah, I think I agree that an explicit input is better. I'll change my PR to match.

[jnsgruk]

One final change here then I think we're good to go:

If channel is unset, we still need to check if LXD is installed, and install with snap install lxd if it isn't. I don't like the idea that canonical/setup-lxd could fail due to not installing LXD ;-)