Re-sign Mac Applications before creating install package

On Apple Silicon, the changes to linking paths done to "relativise" paths in App bundles invalidates the code signature, so we need to recalculate the signatures after the path changes have been performed but before the install package is created. This depends on a new CMake feature introduced in 3.19.

This is necessary due to changes for Apple Silicon that have made Macs much more strict regarding invalid code signatures.