skopeo: switch to 'main' branch
skopeo: switch to 'main' branch

Skopeo has migrated from master to main, so we adjust our branch
accordingly.

Signed-off-by: Bruce Ashfield <284cc737d174a260eec2a2a6324a304f84b1ae0f@gmail.com>

lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization