doc: add type signature of testers.runCommand

doc/build-helpers/testers.chapter.md

@@ -341,6 +341,8 @@ once to get a derivation hash, and again to produce the final fixed output deriv
 
 ## `runCommand` {#tester-runCommand}
 
+`runCommand :: { name, script, stdenv ? stdenvNoCC, hash ? "...", ... } -> Derivation`
+
 This is a wrapper around `pkgs.runCommandWith`, which
 - produces a fixed-output derivation, enabling the command(s) to access the network ;
 - salts the derivation's name based on its inputs, ensuring the command is re-run whenever the inputs changes.