Check for embedded @OUTPUT@s in commands.

mesonbuild/build.py

@@ -1015,9 +1015,10 @@ class CustomTarget:
             else:
                 raise InvalidArguments('Argument %s in "command" is invalid.' % i)
         self.command = final_cmd
-        if self.capture and '@OUTPUT@' in self.command:
-            raise InvalidArguments(
-                '@OUTPUT@ is not allowed when capturing output.')
+        if self.capture:
+            for c in self.command:
+                if isinstance(c, str) and '@OUTPUT@' in c:
+                    raise InvalidArguments('@OUTPUT@ is not allowed when capturing output.')
         if 'install' in kwargs:
             self.install = kwargs['install']
             if not isinstance(self.install, bool):