gitops-pusher supports authenticating with an API key or OAuth
credentials (added in #7393). You shouldn't ever use both of those
together, so we error if both are set.
In tailscale/gitops-acl-action#24, OAuth support is being added to the
GitHub action. In that environment, both the TS_API_KEY and OAuth
variables will be set, even if they are empty values. This causes an
error in gitops-pusher which expects only one to be set.
Update gitops-pusher to check that only one set of environment variables
are non-empty, rather than just checking if they are set.
Updates #7393
Signed-off-by: Will Norris <will@tailscale.com>