Set the PKG_SOURCE_URL using a lazy set to allow evaluating
$(PROJECT_GIT) later. Without this change PKG_SOURCE_URL is evaluated
immediately, before PROJECT_GIT is defined and the download over git is
not working.
Fixes: 6687a2483a ("opkg: use $(PROJECT_GIT), $(AUTORELEASE) and SPDX")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
65b0fe293a