treewide: use runtimeShell instead of stdenv.shell whenever possible

Whenever we create scripts that are installed to $out, we must use runtimeShell
in order to get the shell that can be executed on the machine we create the
package for. This is relevant for cross-compiling. The only use case for
stdenv.shell are scripts that are executed as part of the build system.
Usages in checkPhase are borderline however to decrease the likelyhood
of people copying the wrong examples, I decided to use runtimeShell as well.

pkgs/applications/graphics/autotrace/default.nix

@@ -1,5 +1,6 @@
-{ stdenv, fetchurl, callPackage, libpng12, imagemagick,
-  autoreconfHook, glib, pstoedit, pkgconfig, gettext, gd, darwin }:
+{ stdenv, fetchurl, callPackage, libpng12, imagemagick
+, autoreconfHook, glib, pstoedit, pkgconfig, gettext, gd, darwin
+, runtimeShell }:
 
 # TODO: Figure out why the resultant binary is somehow linked against
 # libpng16.so.16 rather than libpng12.
@@ -51,7 +52,7 @@ stdenv.mkDerivation rec {
     # pstoedit-config no longer exists, it was replaced with pkg-config
     mkdir wrappers
     cat >wrappers/pstoedit-config <<'EOF'
-    #!${stdenv.shell}
+    #!${runtimeShell}
     # replace --version with --modversion for pkg-config
     args=''${@/--version/--modversion}
     exec pkg-config pstoedit "''${args[@]}"