The issue has been fixed in Xcode 12.2 beta 3. The binary frameworks are now properly code signed, as can be seen in the build log.
Note that you have to switch your command-line tools to this specific beta version (12B5035g) and to cleanup your build folder first.
A big thanks to the people who fixed this issue!
Remark: The fix has not been backported to 12.1 GM, the workaround is still required for this version.