automatically produce MSI files for releases

This commit:

- Adds config for building Windows installers
- Adds an action for fetching exe files built by goreleaser
- Adds an action for building Windows installers
- Adds an action for adding MSI files to an existing GH release
- Adds MSI signing to our release flow
- Disables homebrew formula bumping for prereleases
- Allows the release asset copying action to copy windows assets

.github/actions/upload-msi/action.yml

@@ -0,0 +1,5 @@
+name: 'Upload MSI'
+description: 'Upload gh.msi to both homebrew-gh and gh-cli releases'
+runs:
+  using: 'node12'
+  main: 'dist/index.js'