Makefile BUG in edk2 firmware install 4.1.0-rc3

DESTDIR installs end up with wrong paths in JSON files installed to $prefix/share/qemu/firmware. For example, the file:

  50-edk2-x86_64-secure.json

ends up incorrectly with:

  "filename": "/build/qemu/pkg/qemu/usr/share/qemu/edk2-x86_64-secure-code.fd",

instead of the correct:

  "filename": "/usr/share/qemu/edk2-x86_64-secure-code.fd",