github: remove nix-update-flake action

Summary: It needs new credentials to act as a robot account on my behalf, rather
than the 'github-actions' user, as that one can't be approved for security/legal
reasons from Google.

Signed-off-by: Austin Seipp <[email protected]>
Change-Id: I72e303b5625a6417d3444fc65cb79a779a5d9d53