Avoid writing to file in emsdk_env.csh / emsdk_env.sh (#544)

This is an alternative fix for
https://github.com/emscripten-core/emscripten/issues/9090 which recently
came up again after #539.

Tested with bash, tcsh and fish.

emsdk_env.csh

@@ -5,8 +5,6 @@
 # because it won't have any effect then.
 # That is, always run this script with
 #
-#     . ./emsdk_env.csh
-# or
 #     source ./emsdk_env.csh
 #
 # instead of just plainly running with
@@ -18,16 +16,15 @@ set SRC=($_)
 if ("$SRC" == "") then
   set SRC="$0"
 else
-  set SRC="$SRC[2]"
+  set SRC="$SRC[1]"
 endif
 set CURDIR=`pwd`
-set DIR=`dirname "$SRC"`
+setenv DIR `dirname "$SRC"`
 unset SRC
 
 setenv EMSDK_CSH 1
 
-$DIR/emsdk construct_env
-source $DIR/emsdk_set_env.csh
-unset DIR
+eval `$DIR/emsdk construct_env`
+unsetenv DIR
 
 unsetenv EMSDK_CSH