patchShebangs: don't remove ".../bin/env" (fixes NixOS#2146)

FIXME: The linux kernel limits the shebang to 128 bytes
("#define BINPRM_BUF_SIZE 128"), which is too small for
"/nix/store/.../env /nix/store/.../interpreter".

Currently, something like "/usr/bin/env python" is changed into
"/nix/store/<hash>/bin/python" ("env" is removed).

This works in linux systems, but does not work on BSD flavored systems
like OS X since they do not allow another shell script to play a role as
an interpreter. (Interpreters built with nixpkgs are typically shell
script wrappers for the real interpreter binaries.)

The new behaviour is to rewrite "/usr/bin/env python" to
"/nix/store/<hash>/bin/env /nix/store/<hash>/bin/python".

pkgs/build-support/setup-hooks/patch-shebangs.sh

@@ -2,8 +2,9 @@
 # interpreter file names (`#!  /path') to paths found in $PATH.  E.g.,
 # /bin/sh will be rewritten to /nix/store/<hash>-some-bash/bin/sh.
 # /usr/bin/env gets special treatment so that ".../bin/env python" is
-# rewritten to /nix/store/<hash>/bin/python.  Interpreters that are
-# already in the store are left untouched.
+# rewritten to "/nix/store/<hash>/bin/env /nix/store/<hash>/bin/python" (both
+# "env" and "python" get absolute paths). Interpreters that are already in the
+# store are left untouched.
 
 fixupOutputHooks+=('if [ -z "$dontPatchShebangs" ]; then patchShebangs "$prefix"; fi')
 
@@ -35,7 +36,9 @@ patchShebangs() {
                 echo "unsupported interpreter directive \"$oldInterpreterLine\" (set dontPatchShebangs=1 and handle shebang patching yourself)"
                 exit 1
             fi
+            envPath="$(command -v env)"
             newPath="$(command -v "$arg0" || true)"
+            newInterpreterLine="$envPath $newPath $args"
         else
             if [ "$oldPath" = "" ]; then
                 # If no interpreter is specified linux will use /bin/sh. Set
@@ -44,10 +47,9 @@ patchShebangs() {
             fi
             newPath="$(command -v "$(basename "$oldPath")" || true)"
             args="$arg0 $args"
+            newInterpreterLine="$newPath $args"
         fi
 
-        newInterpreterLine="$newPath $args"
-
         if [ -n "$oldPath" -a "${oldPath:0:${#NIX_STORE}}" != "$NIX_STORE" ]; then
             if [ -n "$newPath" -a "$newPath" != "$oldPath" ]; then
                 echo "$f: interpreter directive changed from \"$oldInterpreterLine\" to \"$newInterpreterLine\""