Explicitly set brew install field

Because the automatically generated value is no good when using multiple
builds.

Author: jpcoenen

.goreleaser.yml

@@ -60,7 +60,8 @@ brews:
       owner: secrethub
       name: homebrew-tools
     folder: Formula
-
+    install: |
+      bin.install "bin/secrethub"
     homepage: https://secrethub.io
     description: Command-line interface for SecretHub