[CI] Fix pip upgrade CI (#622)

* try to fix

* Add comments

* Refine grammar

.github/workflows/merge_stage_test.yml

@@ -159,7 +159,8 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - name: Upgrade pip
-        run: pip install pip --upgrade
+        # Windows CI could fail If we call `pip install pip --upgrade` directly.
+        run: python -m pip install pip --upgrade
       - name: Install PyTorch
         run: pip install torch==1.8.1+${{matrix.platform}} torchvision==0.9.1+${{matrix.platform}} -f https://download.pytorch.org/whl/lts/1.8/torch_lts.html
       - name: Build MMEngine from source

.github/workflows/pr_stage_test.yml

@@ -108,7 +108,8 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - name: Upgrade pip
-        run: pip install pip --upgrade
+        # Windows CI could fail If we call `pip install pip --upgrade` directly.
+        run: python -m pip install pip --upgrade
       - name: Install PyTorch
         run: pip install torch==1.8.1+${{matrix.platform}} torchvision==0.9.1+${{matrix.platform}} -f https://download.pytorch.org/whl/lts/1.8/torch_lts.html
       - name: Build MMEngine from source