CI: Use a non-default path to run the upload action

* Don't rely on the action's '$GITHUB_ACTION_PATH' variable being
  the current directory during testing.

.github/workflows/ci.yml

@@ -22,6 +22,9 @@ jobs:
 
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332  # v4.1.7
+        # Don't rely on the action's path being in the current directory
+        with:
+          path: '_action_path'
 
       - name: Set up Python
         uses: actions/setup-python@f677139bbe7f9c59b41e40162b753c062f5d49a3  # v5.2.0
@@ -48,13 +51,13 @@ jobs:
         run: python -m zipfile --list dist/test_package-*.whl
 
       - name: Test upload
-        uses: ./
+        uses: ./_action_path/
         with:
           artifacts_path: dist
           anaconda_nightly_upload_token: ${{ secrets.UPLOAD_TOKEN }}
 
       - name: Test upload that forces removal first
-        uses: ./
+        uses: ./_action_path/
         with:
           artifacts_path: dist
           anaconda_nightly_upload_token: ${{ secrets.UPLOAD_TOKEN }}
@@ -67,7 +70,7 @@ jobs:
           python -m build --outdir ./dist tests/test_package
 
       - name: Test upload with non-main label
-        uses: ./
+        uses: ./_action_path/
         with:
           artifacts_path: dist
           anaconda_nightly_upload_token: ${{ secrets.UPLOAD_TOKEN }}
@@ -81,7 +84,7 @@ jobs:
           python -m build --outdir ./dist tests/test_package
 
       - name: Test upload with multiple labels
-        uses: ./
+        uses: ./_action_path/
         with:
           artifacts_path: dist
           anaconda_nightly_upload_token: ${{ secrets.UPLOAD_TOKEN }}