ci: make workflows fork-PR friendly by removing vars.FLUTTER_VERSION (#447)

Repository variables are not exposed to PRs from forks, so jobs that
reference vars.FLUTTER_VERSION at the container.image level fail to
resolve and the workflow template errors out before any step runs.
Replace it with a setup job that reads the latest published release tag
via the GitHub API using the read-only GITHUB_TOKEN, which is available
in fork-PR contexts.

Co-authored-by: Claude <noreply@anthropic.com>

script/createGitTag.js

@@ -1,20 +1,25 @@
 module.exports = async ({ core, context, github }) => {
-  const { OLD_FLUTTER_VERSION, NEW_FLUTTER_VERSION } = process.env
-
-  if (!OLD_FLUTTER_VERSION) {
-    core.setFailed('Environment variable OLD_FLUTTER_VERSION is required.')
-  }
+  const { NEW_FLUTTER_VERSION } = process.env
 
   if (!NEW_FLUTTER_VERSION) {
     core.setFailed('Environment variable NEW_FLUTTER_VERSION is required.')
-  }
-
-  if (OLD_FLUTTER_VERSION === NEW_FLUTTER_VERSION) {
     return
   }
 
+  // If a tag for this version already exists, do nothing.
+  try {
+    await github.rest.git.getRef({
+      owner: context.repo.owner,
+      repo: context.repo.repo,
+      ref: `tags/${NEW_FLUTTER_VERSION}`,
+    })
+    return
+  } catch (error) {
+    if (error.status !== 404) throw error
+  }
+
   // Create a git tag using the GitHub API instead of the git client to skip SSH/GPG key setup.
-  github.rest.git.createRef({
+  await github.rest.git.createRef({
     owner: context.repo.owner,
     repo: context.repo.repo,
     ref: `refs/tags/${NEW_FLUTTER_VERSION}`,