ios: QMAKE_PRE_LINK Gives Error :Multiple commands produce

Since now that Qt generates "The new build system" with Xcode projects starting with Qt 5.15.6,  QMAKE_PRE_LINK is not usable in  projects. It results in unrelated build errors.

This can be reproduced as follows.
1. Create a new Qt Quick project for iOS Simulator with qmake.
2. Build and make sure it works (no need to run it).
3. Add the following to the qmake project:

QMAKE_PRE_LINK += $$QMAKE_MKDIR ~/foo

4. Rebuild the project.
5. Unrelated build errors regarding "error: Multiple commands produce..."

This can be workaround by switching to  the legacy build system, but that's not ideal.