[.github] add hints to test locally and prevent parsing failures for the first run

[kylos101]

How to test

export GITHUB_ENV=$(mktemp)
SKOPEO_AUTH_FILE=$(mktemp)

cat <<EOF | sudo tee /usr/local/bin/skopeo > /dev/null
#/bin/bash

docker run --rm -v $SKOPEO_AUTH_FILE:/skopeo.auth -e REGISTRY_AUTH_FILE=/skopeo.auth quay.io/skopeo/stable:v1.12.0 "\$@"
EOF

sudo chmod +x /usr/local/bin/skopeo

echo "{}" > $SKOPEO_AUTH_FILE

skopeo login -u oauth2accesstoken --password=foo europe-docker.pkg.dev

Should yield:

time="2023-07-12T18:38:13Z" level=fatal msg="logging into \"europe-docker.pkg.dev\": invalid username/password"

[gitpod-io]

[kylos101]

Nope

edit: I think it is due to this, which writes to the REGISTRY_AUTH_FILE, after successfully logging in