.github: disable fuzzing

This is a partial fix for thesofproject#7709; doc updates still to be done.

Quoting @cujomalainey in
- thesofproject#7699
> I still don't see the benefit here. This branch and IPC3 are mostly
> stable/archived. Hence anything we fuzz on main we should be able to
> cherry-pick down. Nothing new should be landing on this branch so I
> don't see the benefit of continued fuzzing past support on main.

Quoting @andyross in
- thesofproject#7675
> I guess my gut says that fuzzing is a technique for validation of
> new code. It has minimal (but sure, not zero) value on maintenance
> branches that are expected to be protocol-compatible in perpetuity.

Signed-off-by: Marc Herbert <[email protected]>
(cherry picked from commit 3ddd15c)