Maybe this fixes it?

.github/workflows/nextjs_bundle_analysis.yml

@@ -10,13 +10,15 @@ on:
 defaults:
   run:
     # change this if your nextjs app does not live at the root of the repo
-    working-directory: ./beta/
+    working-directory: beta
 
 jobs:
   analyze:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          path: 'beta'
 
       - name: Set up node
         uses: actions/setup-node@v1
@@ -25,6 +27,8 @@ jobs:
 
       - name: Install dependencies
         uses: bahmutov/npm-install@v1
+        with:
+          working-directory: beta
 
       - name: Restore next build
         uses: actions/cache@v2
@@ -32,7 +36,7 @@ jobs:
         env:
           cache-name: cache-next-build
         with:
-          path: .next/cache
+          path: beta/.next/cache
           # change this if you prefer a more strict cache
           key: ${{ runner.os }}-build-${{ env.cache-name }}
 
@@ -49,7 +53,7 @@ jobs:
         uses: actions/upload-artifact@v2
         with:
           name: bundle
-          path: .next/analyze/__bundle_analysis.json
+          path: beta/.next/analyze/__bundle_analysis.json
 
       - name: Download base branch bundle stats
         uses: dawidd6/action-download-artifact@v2
@@ -57,7 +61,7 @@ jobs:
         with:
           workflow: nextjs_bundle_analysis.yml
           branch: ${{ github.event.pull_request.base.ref }}
-          path: .next/analyze/base
+          path: beta/.next/analyze/base
 
       # And here's the second place - this runs after we have both the current and
       # base branch bundle stats, and will compare them to determine what changed.