fix: kubectl install command

[phlg]

The previously used URL doesn't seem to be maintained anymore, as the stable.txt filed used evaluated to v1.31.0.

The URL I replaced them with come from the install documentation : https://kubernetes.io/docs/tasks/tools/install-kubectl-linux/#install-kubectl-binary-with-curl-on-linux

⚠️ For now the amd64 architecture is hardcoded in the URL. Do the images need to cater for other architectures like arm64, or is this good enough ? Nevermind, just saw we had the architecture variable inside, I'll just substitute that instead in a second commit.