`elemental build` needs to be run as root

[mslacken]

As said in the title this command needs to root right to run, but does not check this. Results in following error

ERRO[0000] Build process failed                         
2025/07/02 14:58:53 resolving manifest at uri 'oci://registry.opensuse.org/home/adinov/core/containerfile/release-manifest:0.0.1': reading manifest from source 'registry.opensuse.org/home/adinov/core/containerfile/release-manifest:0.0.1': extracting file from OCI image 'registry.opensuse.org/home/adinov/core/containerfile/release-manifest:0.0.1': unpacking oci image: failed to Lchown "/tmp/release-manifest-unpack-393201595/release_manifest.yaml" for UID 0, GID 0: lchown /tmp/release-manifest-unpack-393201595/release_manifest.yaml: operation not permitted

from which I could see that you have to be root to run this, but checking the uid at the start of elemental build would make this much clearer.

[atanasdinov]

Thanks for pointing this out -- it is something we'd like to address eventually.