root: fix environment setup

1) s/envHook/postHook/
2) patch to not set $p that fixes nested ```nix-shell -p root``` calls

pkgs/applications/science/misc/root/default.nix

@@ -21,6 +21,9 @@ stdenv.mkDerivation rec {
       sha256 = "186i7ni75yvjydy6lpmaplqxfb5z2019bgpbhff1n6zn2qlrff2r";
     })
     ./sw_vers.patch
+
+    # this prevents thisroot.sh from setting $p, which interferes with stdenv setup
+    ./thisroot.patch
   ];
 
   preConfigure = ''

pkgs/applications/science/misc/root/setup-hook.sh

@@ -6,4 +6,4 @@ thisroot () {
     source @out@/bin/thisroot.sh
 }
 
-envHooks+=(thisroot)
+postHooks+=(thisroot)

pkgs/applications/science/misc/root/thisroot.patch

@@ -0,0 +1,15 @@
+diff --git a/config/thisroot.sh b/config/thisroot.sh
+index 85dee20..532cb28 100644
+--- a/config/thisroot.sh
++++ b/config/thisroot.sh
+@@ -15,8 +15,8 @@ drop_from_path()
+       return 1
+    fi
+ 
+-   p=$1
+-   drop=$2
++   local p=$1
++   local drop=$2
+ 
+    newpath=`echo $p | sed -e "s;:${drop}:;:;g" \
+                           -e "s;:${drop};;g"   \