CI: Use cmd prefix on AppVeyor as needed

AppVeyor commands use cmd/batch by default. Specify cmd for lines meant
for cmd to differentiate them from lines meant for ps/PowerShell or
sh/shell/Bash.

appveyor.yml

@@ -8,11 +8,11 @@ install:
   - cmd: C:\projects\obs-studio\CI\install-script-win.cmd
 
 build_script:
-  - call msbuild /m /p:Configuration=%build_config% C:\projects\obs-studio\build32\obs-studio.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-  - call msbuild /m /p:Configuration=%build_config% C:\projects\obs-studio\build64\obs-studio.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
+  - cmd: call msbuild /m /p:Configuration=%build_config% C:\projects\obs-studio\build32\obs-studio.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
+  - cmd: call msbuild /m /p:Configuration=%build_config% C:\projects\obs-studio\build64\obs-studio.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 
 before_deploy:
-  - C:\projects\obs-studio\CI\before-deploy-win.cmd
+  - cmd: C:\projects\obs-studio\CI\before-deploy-win.cmd
 
 deploy_script:
   - ps: Push-AppveyorArtifact "build.zip" -FileName "$(git describe).zip"