Allow passing arbitrary flags to cargo after -- in cargo fuzz build

[tomaka]

The cargo build command has some flags that cargo fuzz build doesn't have.
Two of them are --locked and --frozen, which would be very useful on CI to make sure that the Cargo.lock of the fuzzing targets is up-to-date.
It would be great to add support for these two flags to cargo fuzz build.

[fitzgen]

Chasing support for every cargo flag is something I have done before in cargo foo tools and it is Sysiphian. Not worth the unending effort.

Instead, it is much more maintainable to allow an escape hatch where everything after -- is passed to cargo directly (similar to how we don't copy all of libFuzzer's flags, and allow passing flags to it after -- for cargo fuzz run).