Fix ffi test by using python instead of echo

Some echo implementations don't support `-n` unfortunately.

tests/solidity/cheat/ffi.sol

@@ -11,10 +11,10 @@ contract TestFFI {
 
   function foo(int x) external {
     string[] memory inputs = new string[](3);
-    inputs[0] = "echo";
-    inputs[1] = "-n";
+    inputs[0] = "python3";
+    inputs[1] = "-c";
     // ABI encoded "gm", as a string
-    inputs[2] = "0x00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000002676d000000000000000000000000000000000000000000000000000000000000";
+    inputs[2] = "print('0x00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000002676d000000000000000000000000000000000000000000000000000000000000', end='')";
 
     bytes memory res = Hevm(HEVM_ADDRESS).ffi(inputs);