Run the release Github Action only in the main repo, not in forks

[mccartney]

Why
I have a fork of this repo which I keep on updating. Every time I do that I get an email from Github:

[mccartney/scapegoat] Release workflow run
Release: All jobs have failed

The run fails with this error message:

gpg: no valid OpenPGP data found.

No surprise my fork shouldn't be able to publish a snapshot release of the library.

What
Attempt at having the action run only in the main repo.

[mccartney]

@mwz can you have a look please?
(I cannot assign you as a reviewer)

[mwz]

alternatively you can just disable github actions in your fork, which is usually my preference for any repos I fork