chore: Fork Github PR builds to separate spec (#2740)

Instead of trying to use one single spec that achieves both releasable
builds as well as PR builds, use a different buildspec file for each,
so the PR builds can be made more efficient.

buildspec-pr.yaml

@@ -0,0 +1,11 @@
+version: 0.2
+
+# This buildspec is intended to be used by GitHub PR builds.
+
+phases:
+  install:
+    commands:
+      - /bin/bash ./install.sh
+  build:
+    commands:
+      - /bin/bash ./build.sh

buildspec.yaml

@@ -1,5 +1,7 @@
 version: 0.2
 
+# This buildspec is intended to be run by CodePipeline builds.
+
 phases:
   install:
     commands:
@@ -12,12 +14,7 @@ phases:
       - /bin/bash ./build.sh
   post_build:
     commands:
-      - |
-        if [ -f .BUILD_COMPLETED ]; then
-          if [ -z \"${SKIP_PACK:-}\" ]; then
-            /bin/bash ./pack.sh
-          fi
-        fi
+      - "[ -f .BUILD_COMPLETED ] && /bin/bash ./pack.sh"
 artifacts:
   files:
     - "**/*"