Use a var for cloudflare preview project name

I'm setting up a new CloudFlare account and a new token and trying
to get the pre-merge preview working with wrangler.

I set the var at the org level to "conforma" in the new org and
"enterprise-contract" in the existing org.

What I did to set up the Cloudflare account and token:
* Sign up via https://pages.cloudflare.com/
* Create a token. Use the permissions preset as suggested
  at cloudflare/wrangler-action#233
* Create a GitHub org secret for it called CLOUDFLARE_API_TOKEN
  at the org level, (but limited it to just this repo).
* Create a "pages" project in Cloudflare by clicking "+ Add", then
  "Pages". Name it "conforma", then set a org var called
  CLOUDFLARE_PROJECT_NAME, also limited to just this repo.

.github/workflows/preview.yaml

@@ -74,9 +74,10 @@ jobs:
         id: preview
         env:
           CLOUDFLARE_API_TOKEN: ${{ secrets.CLOUDFLARE_API_TOKEN }}
+          CLOUDFLARE_PROJECT_NAME: ${{ vars.CLOUDFLARE_PROJECT_NAME }}
         run: |
           npm --prefix code ci
-          npm --prefix code exec -- wrangler pages deploy public --project-name enterprise-contract --branch pr-${{ steps.data.outputs.PR_NUMBER }} | tee out.txt
+          npm --prefix code exec -- wrangler pages deploy public --project-name "$CLOUDFLARE_PROJECT_NAME" --branch pr-${{ steps.data.outputs.PR_NUMBER }} | tee out.txt
           grep 'Deployment complete! Take a peek over at ' out.txt | sed -e 's/.*over at /PREVIEW_URL=/' >> $GITHUB_OUTPUT
       - name: Add comment
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1