chore(build): Disable concurrency in pack.sh (aws#2723)

There appear to be a file system race condition during dotnet project
builds where parallel instances race for the same files, resulting in
sporadic build failures due to not being able to access a file (that was
cleaned up by another process already).

Disabling parallelism seems to be the easiest way forward, and also the
cheapest way to validate the root cause was correctly identified.

The `buildspec.yml` will allow skipping pack.sh if the `SKIP_PACK`
environment variable is set.

buildspec.yaml

@@ -12,7 +12,7 @@ phases:
       - /bin/bash ./build.sh
   post_build:
     commands:
-      - "[ -f .BUILD_COMPLETED ] && /bin/bash ./pack.sh"
+      - "[ -f .BUILD_COMPLETED ] && [ -z "${SKIP_PACK:-}" ] && /bin/bash ./pack.sh"
 artifacts:
   files:
     - "**/*"

pack.sh

@@ -16,7 +16,7 @@ scopes=$(lerna ls 2>/dev/null | grep -v "(private)" | cut -d" " -f1 | xargs -n1
 # Run the "cdk-package" script in all modules. For jsii modules, this invokes jsii-pacmak which generates and builds multi-language
 # outputs. For non-jsii module, it will just run "npm pack" and place the output in dist/npm
 # (which is similar to how pacmak outputs it).
-lerna run ${scopes} --sort --stream package
+lerna run ${scopes} --sort --concurrency=1 --stream package
 
 # Collect dist/ from all modules into the root dist/
 for dir in $(find packages -name dist | grep -v node_modules); do