Fix Windows and Mac weekly CI failure: set env correctly (onnx#5582)

### Description
<!-- - Describe your changes. -->
Set env for onnx-weekly correctly.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve? -->
<!-- - If it fixes an open issue, please link to the issue here. -->
Fix Windows and Mac weekly CI failure: set env correctly:
- https://github.com/onnx/onnx/actions/runs/6140254770
- https://github.com/onnx/onnx/actions/runs/6140288376

---------

Signed-off-by: Chun-Wei Chen <jacky82226@gmail.com>

.github/workflows/release_mac.yml

@@ -64,10 +64,9 @@ jobs:
         source workflow_scripts/protobuf/build_protobuf_unix.sh $NUM_CORES $(pwd)/protobuf/protobuf_install
         if [ '${{ github.event_name }}' == 'schedule' ]; then
           sed -i 's/name = "onnx"/name = "onnx-weekly"/' 'pyproject.toml'
-          ONNX_PREVIEW_BUILD=1 python -m build --wheel
-        else
-          python -m build --wheel
+          export ONNX_PREVIEW_BUILD=1
         fi
+        python -m build --wheel
 
         for file in dist/*.whl; do
           python -m pip install --upgrade $file;

.github/workflows/release_win.yml

@@ -72,10 +72,9 @@ jobs:
         if ('${{ github.event_name }}' -eq 'schedule') {
           echo "Build weekly PyPI package"
           (Get-Content -Path 'pyproject.toml') | ForEach-Object { $_ -replace 'name = "onnx"', 'name = "onnx-weekly"' } | Set-Content -Path 'pyproject.toml'
-          ONNX_PREVIEW_BUILD=1 python -m build --wheel
-        } else {
-          python -m build --wheel
+          set ONNX_PREVIEW_BUILD=1
         }
+        python -m build --wheel
         Get-ChildItem -Path dist/*.whl | foreach {python -m pip install --upgrade $_.fullname}
 
     - name: Test the installed wheel