Fix mac builds (#376)

dist/index.js

@@ -3505,7 +3505,7 @@ class ImageTag {
             case 'linux':
                 return 'ubuntu';
             default:
-                throw new Error('The Operating System of this runner is not yet supported.');
+                return '';
         }
     }
     static getTargetPlatformToTargetPlatformSuffixMap(platform, version) {

src/model/image-tag.ts

@@ -60,7 +60,7 @@ class ImageTag {
       case 'linux':
         return 'ubuntu';
       default:
-        throw new Error('The Operating System of this runner is not yet supported.');
+        return '';
     }
   }