CI: Use stable toolchain for test workflows

We need stable because Criterion dependency MSRVs exceed ours. This is
fine in tests - the Check phase already uses our MSRV to assert that
the library's source is ok on the pinned version.

Author: the-mikedavis

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
       - name: Install MSRV toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: "${{ env.MSRV }}"
+          toolchain: stable
 
       - uses: Swatinem/rust-cache@v2