ci(build): 🎡 suppress virtualenv activation error output on non-POSIX runners

Silence stderr when sourcing the POSIX venv activation and fall back to the Windows activation to reduce noisy errors in the build workflow.

- redirect `source .venv/bin/activate` stderr to `/dev/null` so failures on non-POSIX shells don't emit errors
- apply the same change in the "Build executable" step for consistency

.github/workflows/build.yml

@@ -45,7 +45,7 @@ jobs:
       shell: bash
       run: |
         grep -v -- '-e src/rotator_library' requirements.txt > temp_requirements.txt
-        source .venv/bin/activate || . .venv/Scripts/activate
+        source .venv/bin/activate 2>/dev/null || source .venv/Scripts/activate
         uv pip install -r temp_requirements.txt
         uv pip install pyinstaller
         uv pip install -e src/rotator_library
@@ -73,7 +73,7 @@ jobs:
     - name: Build executable
       shell: bash
       run: |
-        source .venv/bin/activate || . .venv/Scripts/activate
+        source .venv/bin/activate 2>/dev/null || source .venv/Scripts/activate
         python src/proxy_app/build.py
 
     - name: Ensure PyInstaller cache directory exists