nixos/tests/geth: fix api check

Since https://github.com/ethereum/go-ethereum/pull/21686 the `chainId`
API returns an error when the chain is not synced. So use
`blockNumber` as a basic function test.

nixos/tests/geth.nix

@@ -31,7 +31,7 @@ import ./make-test-python.nix ({ pkgs, ... }: {
     machine.wait_for_open_port(18545)
 
     machine.succeed(
-        'geth attach --exec "eth.chainId()" http://localhost:8545 | grep \'"0x0"\' '
+        'geth attach --exec eth.blockNumber http://localhost:8545 | grep \'^0$\' '
     )
 
     machine.succeed(