fix: cleanup macOS packaging

Don't allow specifying target
Add `set -e` to shell scripts
Don't try and package on every PR

Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>

.github/workflows/build-pkg.yml

@@ -4,9 +4,6 @@ on:
     paths:
       - .github/workflows/build-pkg.yml
       - package/scripts
-  pull_request:
-    branches:
-      - master
   release:
     types:
       - published

package/scripts/postinstall

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -e
 
 # verify the files exist
 tmp_brew="$2"
@@ -8,9 +9,7 @@ if [[ ! -d "${tmp_brew:?}" ]]; then
 fi
 
 # pick the correct target
-if [[ "$3" != "/" ]]; then
-  target=$3
-elif [[ $(uname -m) == "x86_64" ]];then
+if [[ $(uname -m) == "x86_64" ]];then
   target="/usr/local"
 else
   target="/opt/Homebrew"