fix: fetch release branch ref as head (#2566)

.github/workflows/update-changeset-prs.yaml

@@ -5,10 +5,6 @@ on:
     branches:
       - 'changeset-release/**'
 
-env:
-  TURBO_SCM_BASE: ${{ github.event.before }}
-  TURBO_SCM_HEAD: ${{ github.sha }}
-
 # enforce only one release action per release branch at a time
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -61,7 +57,23 @@ jobs:
         # We want to commit the yarn.lock changes
         run: yarn install
 
+      - name: Get latest commit on release branch
+        id: get-latest-commit
+        uses: actions/github-script@v6
+        with:
+          script: |
+            const { data } = await github.rest.repos.getBranch({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              branch: '1.2.x'
+            });
+            console.log('Latest commit SHA:', data.commit.sha); 
+            core.setOutput('main_sha', data.commit.sha);
+
       - name: Clean export dynamic
+        env:
+          TURBO_SCM_BASE: ${{ steps.get-latest-commit.outputs.main_sha }}
+          TURBO_SCM_HEAD: ${{ github.sha }}
         # We want a clean dynamic folder
         run: yarn run export-dynamic:clean --concurrency=75% --affected