fix(ci): Use locked version of llvm-cov

A transitive dependency of llvm-cov was published 2025-10-23 that is
incompatible with our current Rust version, 1.87. This adds the `--locked` flag
so that the tool can successfully install.

Author: iinuwa

.github/workflows/rust-test.yml

@@ -97,7 +97,7 @@ jobs:
         uses: Swatinem/rust-cache@f0deed1e0edfc6a9be95417288c0e1099b1eeec3 # v2.7.7
 
       - name: Install cargo-llvm-cov
-        run: cargo install cargo-llvm-cov --version 0.5.38
+        run: cargo install cargo-llvm-cov --version 0.5.38 --locked
 
       - name: Generate coverage
         run: cargo llvm-cov --all-features --lcov --output-path lcov.info --ignore-filename-regex "crates/bitwarden-api-"