Add --no-self-update to debug step in Github workflow

To be consistent with what cargo-msrv does.

.github/workflows/msrv.yml

@@ -68,7 +68,7 @@ jobs:
       - name: install_rust
         uses: dtolnay/rust-toolchain@stable
       - name: debug_manual_install
-        run: rustup install --profile minimal 1.78.0
+        run: rustup install --no-self-update --profile minimal 1.78.0
       - name: debug_manual_install_two
         run: rustup install --profile minimal 1.78.0