test(e2e): refer to correct env variable for Playwright version in wo…

…rkflow file

The key here should be made from env, and not vars.

See: https://github.com/sanity-io/sanity/actions/caches

.github/workflows/e2e.yml

@@ -64,7 +64,7 @@ jobs:
         uses: actions/cache@v4
         with:
           path: ~/.cache/ms-playwright
-          key: playwright-browsers-${{ vars.PLAYWRIGHT_VERSION }}
+          key: playwright-browsers-${{ env.PLAYWRIGHT_VERSION }}
 
       - name: Install Playwright Browsers
         if: steps.cache-playwright-browsers.outputs.cache-hit != 'true'