We were replacing their install-commands with %{make-install} (as
their existing install-commands assume DESTDIR is set in the
environment) but that doesn't work because they are 'manual' type
elements and %{make-install} isn't set anywhere.

Fixes this error:

    Error loading pipeline: /mnt/build/src/buildstream/buildstream/data/projectconfig.yaml [line 53 column 2]: Dictionary did not contain expected key 'make-install'