CI: fix workspace doc build

[tarcieri]

The crypto crate is linking old versions of the trait crates, and that's causing collisions when building the workspace rustdoc:

https://github.com/RustCrypto/traits/actions/runs/11112631698/job/30875140095

warning: output filename collision.
The lib target `aead` in package `aead v0.6.0-rc.0 (/Users/tony/src/RustCrypto/traits/aead)` has the same output filename as the lib target `aead` in package `aead v0.5.2`.
Colliding filename is: /Users/tony/src/RustCrypto/traits/target/doc/aead/index.html
The targets should have unique names.
This is a known bug where multiple crates with the same name use
the same path; see <https://github.com/rust-lang/cargo/issues/6313>.
warning: output filename collision.
The lib target `cipher` in package `cipher v0.5.0-pre.7 (/Users/tony/src/RustCrypto/traits/cipher)` has the same output filename as the lib target `cipher` in package `cipher v0.4.4`.
Colliding filename is: /Users/tony/src/RustCrypto/traits/target/doc/cipher/index.html
The targets should have unique names.
This is a known bug where multiple crates with the same name use
the same path; see <https://github.com/rust-lang/cargo/issues/6313>.

This changes the CI config to exclude it from the rustdoc build for now, since everything else is on a prerelease series.

We should probably bump crypto to link the latest prereleases soon, but for now this gets CI green again.