run the downstream ci job under bash

The default shell on Windows runners is PowerShell, where 'r' is an alias
for Invoke-History; 'R CMD INSTALL' was parsed as 'Invoke-History CMD
INSTALL' and failed before the job did anything useful.

Author: kevinushey

.github/workflows/R-CMD-check.yaml

@@ -63,6 +63,12 @@ jobs:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
       VERBOSE: 1
 
+    # the default shell on Windows runners is PowerShell, which aliases 'r'
+    # to Invoke-History and so mangles 'R CMD INSTALL'
+    defaults:
+      run:
+        shell: bash
+
     steps:
       - uses: actions/checkout@v3