Disable library validation

[pjrobertson]

Allows loading un-signed plugins

Test pull request to have GH Actions build a signed copy to debug.

Related to #2634 (comment)

[pjrobertson]

OK, so this builds fine and verifies fine:

/Downloads ❯❯❯ spctl --assess --verbose --type open --context context:primary-signature "Quicksilver 1.6.1.dmg"
Quicksilver 1.6.1.dmg: accepted
source=Notarized Developer ID
/Quicksilver ❯❯❯ spctl --assess --verbose --type open --context context:primary-signature "Quicksilver.app"
Quicksilver.app: accepted
source=Notarized Developer ID

Edit: and of course, the main thing is this signed Quicksilver.app can load non-signed plugins.

Since this is such a quick and easy change, and removes the need for us to re-download and re-build all other plugins, here's my proposal:

1. This gets merged into 2.0, we release 2.0 'as is' now that most plugins have already been rebuilt for M1
2. Once 2.0 is out, we can take our time to code-sign all other outstanding plugins and release them post 2.0 release
3. For 2.1, we re-enable Library Validation, that means only plugins signed by us can be loaded.

In my mind, this + the Donation PR should be all we need to get 2.0 out. There are a few outstanding plugins, but I'm not that concerned - they're mostly ones that don't work now anyway.