Fixed crash loop on renamed registration file

[dermalikmann]

When the user (e.g. "Me") decides to rename the file with the registration info, this could end in two ways, depending if $GITEA_RUNNER_REGISTRATION_TOKEN_FILE and $GITEA_RUNNER_REGISTRATION_TOKEN are set:

1. If one or both env var(s) is/are set, the runner will re-register on every restart of the container, resulting in numerous "offline" runners in gitea.
2. If neither env var is set, the runner will run into a crash loop, as it expects the .runner file or a token-source, neither of which are available.

PS: I was not able to test this as I ran into issues every step on the way to building the image.

[sebthom]

Thanks!