symlinkJoin: fix "bash: Argument list too long" error

When the `paths` argument is too big `symlinkJoin` will fail with:

```
while setting up the build environment: executing '/nix/store/rm1hz1lybxangc8sdl7xvzs5dcvigvf7-bash-4.4-p23/bin/bash': Argument list too long
```

This is fixed by passing `paths` as a file instead of as an
environment variable.

pkgs/build-support/trivial-builders.nix

@@ -255,11 +255,14 @@ rec {
          }:
     let
       args = removeAttrs args_ [ "name" "postBuild" ]
-        // { inherit preferLocalBuild allowSubstitutes; }; # pass the defaults
+        // {
+          inherit preferLocalBuild allowSubstitutes;
+          passAsFile = [ "paths" ];
+        }; # pass the defaults
     in runCommand name args
       ''
         mkdir -p $out
-        for i in $paths; do
+        for i in $(cat $pathsPath); do
           ${lndir}/bin/lndir -silent $i $out
         done
         ${postBuild}