brew-services: don't hardcode launchctl path

This change allows users to install the `launchctl` wrapper in the
`reattach-to-user-namespace` formula which makes the command work in
Tmux and other non-standard environments.

Closes Homebrew/homebrew#26609.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Contributions/cmd/brew-services.rb

@@ -94,7 +94,7 @@ module ServicesCli
     def bin; "brew services" end
 
     # Path to launchctl binary.
-    def launchctl; "/bin/launchctl" end
+    def launchctl; which("launchctl") end
 
     # Wohoo, we are root dude!
     def root?; Process.uid == 0 end