[GitHub] Remove redundant cache key prefix (#76914)

Remove the redundant sccache cache key prefix.
This prefix is already added by the ccache action, which results in
cache keys like "sccache-sccache-ubuntu-...".

See the following source lines as proof:

https://github.com/hendrikmuhs/ccache-action/blob/2a51777f6f64b7b7bea213601acba8f5f4fdbe03/src/restore.ts#L22-L23