build: LOAD and PUSH set in the environment are ignored, so a local load pushes instead #4499

Setting LOAD=1 PUSH=0 in the environment has no effect on make image. The build still pushes instead of loading into the local docker store. The reason is that hack/common-envs.mk assigns PUSH := 1 and LOAD := 0 with plain :=. Make lets a command-line variable override that, but an environment variable loses to it. So make image LOAD=1 PUSH=0 works, while LOAD=1 PUSH=0 make image quietly pushes.

I checked it with make -n -C packages/system/cozystack-controller image. From the environment it expands to --push=1 --load=0, from the command line to --push=0 --load=1.

This is older than #4498. After #4498 it gets worse: the same environment call becomes a push for both linux/amd64 and linux/arm64, because the multi-arch default applies whenever LOAD is not 1.

The fix is ?= for both, like BUILDER, PLATFORM and SBOM already have. The OCI_EXPORT_DIR block that forces both to 0 must stay after that, so the fork export still wins.