ci(cache): override rust version in cache factory for MSRV cache (#3248)

Previously, we forgot to override the default Rust version used to the newly installed one for the MSRV cache. This caused `swatinem/rust-cache` to compute the wrong cache key. See https://github.com/libp2p/rust-libp2p/actions/runs/3697491863/jobs/6262555434#step:5:21 for example.

This caused all pull-requests to not actually find this cache and rebuild everything from scratch: https://github.com/libp2p/rust-libp2p/actions/runs/3699757580/jobs/6267459677#step:6:21

With this patch, we are now considering the correct Rust version: https://github.com/thomaseizinger/rust-libp2p/actions/runs/3699821930/jobs/6267596081#step:5:21

Author: thomaseizinger

.github/workflows/cache-factory.yml

@@ -41,6 +41,7 @@ jobs:
       - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
         with:
           toolchain: ${{ matrix.rust }}
+          override: true
 
       - uses: Swatinem/rust-cache@359a70e43a0bb8a13953b04a90f76428b4959bb6 # v2.2.0
         with:
@@ -63,6 +64,7 @@ jobs:
       - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
         with:
           toolchain: stable
+          override: true
 
       - uses: Swatinem/rust-cache@359a70e43a0bb8a13953b04a90f76428b4959bb6 # v2.2.0
         with: