Prevent concurrent deployments

[dscho]

Changes

Let's avoid having multiple deployment runs stumble over each other's toes.

Context

GitHub Pages' sites are singletons and therefore we should use the recommended pattern to prevent multiple deployments to run at the same time; Instead, we want to queue them all up.

Otherwise the same might happen as in this deployment, which was triggered by merging a PR while the previous deployment (triggered by merging another PR) was still running:

⨂ deploy
   HttpError: Deployment request failed for ca3b63f due to in progress deployment. Please cancel 5b1041a first or wait for it to complete.
      at /home/runner/work/_actions/actions/deploy-pages/v4/node_modules/@octokit/request/dist-node/index.js:124:1
      at processTicksAndRejections (node:internal/process/task_queues:95:5)
      at createPagesDeployment (/home/runner/work/_actions/actions/deploy-pages/v4/src/internal/api-client.js:125:1)
      at Deployment.create (/home/runner/work/_actions/actions/deploy-pages/v4/src/internal/deployment.js:74:1)
      at main (/home/runner/work/_actions/actions/deploy-pages/v4/src/index.js:30:1)

⨂ deploy
   Error: Failed to create deployment (status: 400) with build version ca3b63f. Request ID 2460:3B264E:2D28F10:5686FAF:66F57B0B Responded with: Deployment request failed for ca3b63f due to in progress deployment. Please cancel 5b1041a first or wait for it to complete.

[dscho]

I verified that this is working correctly by first pushing this branch to my gh-pages, which triggered this run on push, then immediately triggering another run via workflow_dispatch, which is now waiting:

[dscho]

I'll just go ahead and merge this.