update.nix: Run update scripts in parallel

To make updating large attribute sets faster, the update scripts
are now run in parallel.

Please note the following changes in semantics:

- The string passed to updateScript needs to be a path to an executable file.
- The updateScript can also be a list: the tail elements will then be passed
  to the head as command line arguments.

1 files changed, 1 insertions, 0 deletions

diff --git a/pkgs/development/libraries/safefile/default.nix b/pkgs/development/libraries/safefile/default.nix
index 159bad4c68e..d09e45a2d1e 100644
--- a/pkgs/development/libraries/safefile/default.nix
+++ b/pkgs/development/libraries/safefile/default.nix
@@ -13,6 +13,7 @@ stdenv.mkDerivation rec {
 
   passthru = {
     updateScript = ''
+      #!${stdenv.shell}
       cd ${toString ./.}
       ${toString path}/pkgs/build-support/upstream-updater/update-walker.sh default.nix
     '';