cmd/gitops-pusher: fix logic for checking credentials

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>
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