Scan Windows release package using Microsoft Defender

Things aren't perfect, but they're the best we can do given what we're
left to work with. For example:

- We cannot update the virus signatures, because that command simply
fails, despite being copy-and-paste'd from Microsoft's documentation.

- When queried, Windows reports its virus signature age is
4,294,967,295 days old, which, on geological time scales, pre-dates
even the Pleistocene era by 9.7 million years.

Despite these issues, my hunch is that the signatures are current as of
the date when the VM's base-image was generated; and because we're in
the Holocene era of zero-day network threats, my guess is GitHub is
re-generating images at least weekly if not more frequently (and sure
enough, Defender catches the EICAR dummy virus.

Note that we're not using Start-MpScan because it does not provide any
useful output on status of scans (indeed!), so I've moved that here into
the commit message.

.github/workflows/linux.yml

@@ -142,7 +142,7 @@ jobs:
           # Create tarball
           tar -cJf "dosbox-staging-linux-$VERSION.tar.xz" "dosbox-staging-linux-$VERSION"
 
-      - name: AV scan
+      - name: Clam AV scan
         run: |
           set -x
           sudo apt-get install clamav > /dev/null

.github/workflows/macos.yml

@@ -144,7 +144,7 @@ jobs:
               -srcfolder dist \
               -ov -format UDZO "dosbox-staging-macOS-${{ env.VERSION }}.dmg"
 
-      - name: AV scan
+      - name: Clam AV scan
         run: |
           set -x
           brew install clamav > /dev/null

.github/workflows/windows.yml

@@ -169,6 +169,18 @@ jobs:
           sed -i "s|%GITHUB_REPO%|$GITHUB_REPOSITORY|"       dest/README.txt
           # Create dir for zipping
           mv dest dosbox-staging-windows-${{ env.VERSION }}
+
+      - name: Windows Defender AV Scan
+        shell: powershell
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $dosboxDir = "${{ github.workspace }}/dosbox-staging-windows-${{ env.VERSION }}"
+          & 'C:\Program Files\Windows Defender\MpCmdRun.exe' -Scan -ScanType 3 -DisableRemediation -File $dosboxDir
+          if( $LASTEXITCODE -ne 0 ) {
+              Get-Content -Path $env:TEMP\MpCmdRun.log
+              Throw "Exit $LASTEXITCODE : Windows Defender found an issue"
+          }
+
       - name: Upload package
         uses: actions/upload-artifact@master
         with: