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

setup-windows-certificate.ps1

@@ -0,0 +1,9 @@
+$scriptPath = split-path -parent $MyInvocation.MyCommand.Definition
+
+$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
+$headers.Add("Authorization", "token $env:DESKTOP_CERT_TOKEN")
+$headers.Add("Accept", 'application/vnd.github.v3.raw')
+
+Invoke-WebRequest 'https://api.github.com/repos/desktop/desktop-secrets/contents/windows-certificate.pfx' `
+              -Headers $headers `
+              -OutFile "$scriptPath\windows-certificate.pfx"