CI: Reduce the number of packages that are updated for wasip2 MSRV test (#825)

Author: briansmith

.github/workflows/tests.yml

@@ -391,10 +391,11 @@ jobs:
           wget -O - $URL | tar -xJ --strip-components=1 -C ~/.cargo/bin
           wasmtime --version
       - uses: Swatinem/rust-cache@v2
+      # TODO(MSRV-1.87): Remove this step.
       - name: Generate MSRV-compatible Cargo.lock
         env:
           CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS: "fallback"
-        run: cargo update
+        run: cargo update -p wasip2
       - run: cargo test --target wasm32-wasip2
 
   # TODO: enable after pre-built std will be provided by Rust