ci: Only sync channel from upstream

Author: tomasol

.github/workflows/sync-flake-lock.yml

@@ -26,7 +26,9 @@ jobs:
       - name: Sync flake.lock and rust-toolchain.toml from upstream
         run: |
           curl https://raw.githubusercontent.com/obeli-sk/obelisk/refs/heads/latest/flake.lock -o flake.lock
-          curl https://raw.githubusercontent.com/obeli-sk/obelisk/refs/heads/latest/rust-toolchain.toml -o rust-toolchain.toml
+          UPSTREAM=https://raw.githubusercontent.com/obeli-sk/obelisk/refs/heads/latest/rust-toolchain.toml
+          CHANNEL=$(curl -fsSL "$UPSTREAM" | grep '^channel =')
+          sed -i "s/^channel =.*/$CHANNEL/" rust-toolchain.toml
 
       - uses: nixbuild/nix-quick-install-action@v34
         with: