Allow buildInputs to be regular files

If a build input is a regular file, use it as a setup hook. This makes
setup hooks more efficient to create: you don't need a derivation that
copies them to $out/nix-support/setup-hook, instead you can use the
file as is.

pkgs/stdenv/generic/setup.sh

···
 
     eval $var="'${!var} $pkg '"
 
+    if [ -f $pkg ]; then
+        source $pkg
+    fi
+
     if [ -f $pkg/nix-support/setup-hook ]; then
         source $pkg/nix-support/setup-hook
     fi