Add GitHub Action to update image tag used when updating metacpan-web

[ssoriche]

Action should:

1. checkout the metacpan-k8s repository alongside the metacpan-web repository
2. issue the kustomize edit set image "metacpan/metacpan-web:latest" "metacpan/metacpan-web:${{ github.sha }}" command
3. commit and push the resulting change back to the metacpan-k8s repository onto main

Reference for checking out two repositories in GitHub Actions: https://github.com/actions/checkout?tab=readme-ov-file#Checkout-multiple-repos-side-by-side

[haarg]

We need new deployments to k8s to send a purge of the key content_type=text/html to Fastly before this can be automated.