Skip postgresql if it's already installed

As part of the `script/bootstrap` script, we install the `postgresql`
package. If the developer has already installed `Postgres.app` (which
has some [useful
features](https://chrisrbailey.medium.com/postgres-on-mac-skip-brew-use-postgres-app-dda95da38d74))
they might run into issues.

So, let's amend the Brewfile to only install `postgresql` if the `pgsql`
command isn't already available.

Brewfile

@@ -1,5 +1,5 @@
 brew "nodenv"
-brew "postgresql"
+brew "postgresql" unless system "which -s psql"
 brew "rbenv"
 brew "shellcheck"
 brew "yarn"