Set bash shell on Windows (pytorch#128854)

Attempt to fix the missing python3 command on the new Windows AMI https://github.com/pytorch/pytorch/actions/runs/9551494945/job/26325922503.  I added the logic to copy python to python3 to make the command available, it worked with the previous AMI, but start to fail now and the cause is not clear (maybe it's not the AMI, but a new GitHub runner version)
Pull Request resolved: pytorch#128854
Approved by: https://github.com/kit1980, https://github.com/malfet, https://github.com/atalman

Author: huydhn

.github/workflows/_win-build.yml

@@ -47,6 +47,9 @@ jobs:
     timeout-minutes: 240
     outputs:
       test-matrix: ${{ steps.filter.outputs.test-matrix }}
+    defaults:
+      run:
+        shell: bash
     steps:
       # Duplicated in win-test because this MUST go before a checkout
       - name: Enable git symlinks on Windows and disable fsmonitor daemon
@@ -89,6 +92,7 @@ jobs:
 
       - name: Parse ref
         id: parse-ref
+        shell: bash
         run: python3 .github/scripts/parse_ref.py
 
       - name: Get workflow job id

.github/workflows/_win-test.yml

@@ -41,6 +41,9 @@ jobs:
       fail-fast: false
     runs-on: ${{ matrix.runner }}
     timeout-minutes: ${{ matrix.mem_leak_check == 'mem_leak_check' && 600 || inputs.timeout-minutes }}
+    defaults:
+      run:
+        shell: bash
     steps:
       # Duplicated in win-build because this MUST go before a checkout
       - name: Enable git symlinks on Windows and disable fsmonitor daemon
@@ -224,6 +227,7 @@ jobs:
 
       - name: Parse ref
         id: parse-ref
+        shell: bash
         run: python3 .github/scripts/parse_ref.py
 
       - name: Uninstall PyTorch