pkgs.runCommand: passAsFile (buildCommand can be very long)

Close #15803. This avoids the error:

while setting up the build environment: executing
‘/nix/store/7sb42axk5lrxqz45nldrb2pchlys14s1-bash-4.3-p42/bin/bash’:
Argument list too long

Note: I wanted to make it optional based on buildCommand length,
but that seems pointless as I'm sure it's less performant.

Amended by vcunat:
https://github.com/NixOS/nixpkgs/pull/15803#issuecomment-224841225