Add simple install test for PRs

[csantanapr]

there should be a GitHub action that tests the install of the plugins, this simple test should catch simple errors like on reported on Slack today

Hello Knative, long time no see. I’m really wanting to check out the new quickstart system but it looks like I might have caught it at a bad time or might be doing something wrong. When I try to install it I get:
❯ brew install knative-sandbox/kn-plugins/quickstart
==> Tapping knative-sandbox/kn-plugins
Cloning into '/usr/local/Homebrew/Library/Taps/knative-sandbox/homebrew-kn-plugins'...
remote: Enumerating objects: 203, done.
remote: Counting objects: 100% (203/203), done.
remote: Compressing objects: 100% (152/152), done.
remote: Total 203 (delta 126), reused 66 (delta 43), pack-reused 0
Receiving objects: 100% (203/203), 50.63 KiB | 4.22 MiB/s, done.
Resolving deltas: 100% (126/126), done.
Error: Invalid formula: /usr/local/Homebrew/Library/Taps/knative-sandbox/homebrew-kn-plugins/[email protected]
No available formula with the name "[email protected]". Did you mean [email protected]?
In formula file: /usr/local/Homebrew/Library/Taps/knative-sandbox/homebrew-kn-plugins/[email protected]
Expected to find class EventAT11, but only found: Event.
Error: Cannot tap knative-sandbox/kn-plugins: invalid syntax in tap!

[sbdtu5498]

/assign