ci: Pin dependency versions for MSRV

Author: bluss

.github/workflows/ci.yml

@@ -26,6 +26,10 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ matrix.rust }}
+      - name: Pin versions for MSRV
+        if: "${{ matrix.rust == '1.64.0' }}"
+        run: |
+          cargo update -p regex --precise 1.8.4
       - name: Build
         run: cargo build --verbose
       - name: Run tests