Feature request: Allow KREW_PATH to contain multiple paths

When running krew in a oci build environment image we'd like to pre-seed a system-wide set of krew plugins but allow users to also install plugins for their own user. This way CI pipelines can rely on a standard set of plugins while developers can install additional services locally when testing.

If the KREW_PATH supported the following syntax and always used the first path found then this would become possible:

export KREW_PATH=/home/$USER/.krew:/opt/krew/

Windows might need a different syntax with ; instead of :.

Related issues: https://github.com/kubernetes-sigs/krew/issues/623