CI fix, part II: kube

[apostasie]

This is fixing the recent kube testing bustage.

NOTE that this PR is on-top of #606 as I needed a working CI environment as a starting point to debug. We can either merge this and close 606, or merge 606 first, then I will rebase here.

As far as I can tell, the issues with kube were:

1. fallout of #603 that did miss a bunch of strings
2. containerd-nydus-grpc was built dynamically on the host, but copied into the kind control plane container that could (and does) have a different libc
3. the rig was tightly coupled with github actions, making it really hard to test locally. Specifically, we depended upon engineerd/setup-kind which has been untouched in two years and is using deprecated features from github action anyhow

What this PR does (in commit "Fix kube test bustage"):

• add a new make static target so that we get truly static bin/containerd-nydus-grpc binaries that we can safely copy into the kind control plane
• finish replacing /var/lib/containerd/io.containerd.snapshotter.v1.nydus (follow-up to #603)
• pin delve version so that we don't have the go version problem anymore in the future (mentioned in #606)
• bump nerdctl version
• replace the entire github-action-coupled rig setup by a kind.sh shell script (under tests/helpers) that takes care of setting everything up, so that we can also use that locally / outside of github actions

CI is now green and both PRs are ready to review.

Hope this is helpful.

[apostasie]

@imeoer going to rebase this ASAP so that we can consider just the Kube fixes

[apostasie]

@imeoer rebased - one commit left - CI is green.

I just added a bunch of inline comments to facilitate review. PTAL at your convenience and LMK if you want anything changed in this.

Thanks for your help!