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

pkgs/build-support/trivial-builders.nix

@@ -8,6 +8,7 @@ rec {
   runCommand = name: env: buildCommand:
     stdenv.mkDerivation ({
       inherit name buildCommand;
+      passAsFile = [ "buildCommand" ];
     } // env);
 
 

pkgs/stdenv/generic/setup.sh

@@ -827,6 +827,10 @@ showPhaseHeader() {
 
 
 genericBuild() {
+    if [ -f "$buildCommandPath" ]; then
+        . "$buildCommandPath"
+        return
+    fi
     if [ -n "$buildCommand" ]; then
         eval "$buildCommand"
         return