ci: Do not use --locked in cargo-careful installation

RalfJung/cargo-careful#18

```
error[E0463]: can't find crate for `proc_macro`
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/lib.rs:123:1
    |
123 | extern crate proc_macro;
    | ^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

error[E0635]: unknown feature `proc_macro_span`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/lib.rs:92:13
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |             ^^^^^^^^^^^^^^^

error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^
```

.github/workflows/ci.yml

@@ -77,6 +77,7 @@ jobs:
       - uses: taiki-e/cache-cargo-install-action@v1
         with:
           tool: cargo-careful
+          locked: false # TODO: remove once https://github.com/RalfJung/cargo-careful/pull/18 is released.
         if: startsWith(matrix.rust, 'nightly')
       - run: cargo test --all --all-features
       - run: cargo careful test --all --all-features