chore: pr-build failing with unsafe-perm is not a valid npm option (aws#28068)

We switched the images used to build PRs to `jsii/superchain:bullseye-slim-node18` which comes with Node.js 18 instead of Node.js 16. This Node.js upgrade also includes an upgraded version of npm, and in recent npm versions, the unsef-perm option is not required anymore. This change simply removes the now redundant command.

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Author: mrgrain

buildspec-pr.yaml

@@ -14,9 +14,6 @@ phases:
       # this also takes care of launching the docker daemon.
       - /root/ecr-proxy/start.sh
 
-      # CodeBuild always runs as root, allow npm to operate as such
-      - npm config set unsafe-perm true
-
       # Install yarn if it wasn't already present in the image
       - yarn --version || npm -g install yarn