Remove nrwl/nx-set-shas from CI

[unmultimedio]

Summary

• Removes the nrwl/nx-set-shas action from the CI workflow
• Adds a new last-successful-commit command (using appcmd) that calls the GitHub Actions API directly to find the last successful run SHA for a given workflow and branch
• Updates the CI step to use last-successful-commit + changed-plugins instead of relying on the action output

[pkwarren]

Instead of a new go command, could we use the gh CLI to accomplish the same thing:

gh api "repos/${{ github.repository }}/actions/workflows/ci.yml/runs?branch=${{ github.ref_name }}&status=success&per_page=1" --jq '.workflow_runs[0].head_sha'`

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