chore(ci): Ensure lockfile is respected during MSRV testing

As a hack in cargo-hack, it doesn't respect lockfiles when doing MSRV
testing unless `--locked` is passed in.
This adds that so we make sure we don't run into problems with newer,
MSRV-imcompatible dependencies come out that break our build.

See
- https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/gix-ref.20CI.20error/near/423319798
- taiki-e/cargo-hack#234
- taiki-e/cargo-hack#236

.github/workflows/main.yml

@@ -267,4 +267,4 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - uses: taiki-e/install-action@cargo-hack
-    - run: cargo hack check --all-targets --rust-version --workspace --ignore-private
+    - run: cargo hack check --all-targets --rust-version --workspace --ignore-private --locked