Previously it was awkward to use the runCommand-variants with
passAsFile as a double definition of passAsFile would potentially
break runCommand: passAsFile would overwrite the previous definition,
defeating the purpose of setting it in runCommand in the first place.
This is now fixed by concatenating the [ "buildCommand" ] list with
one the one from env, if present.
Adjust buildEnv where passAsFile = null; was passed in some cases,
breaking evaluation since it'd evaluate to [ "buildCommand" ] ++ null.
b398d00903