Avoid file creation/deletion races in emsdk_env.sh by using a temporary file(#307)

Fixes https://github.com/emscripten-core/emscripten/issues/9090.

emsdk_env.sh

@@ -14,7 +14,6 @@
 #     ./emsdk_env.sh
 #
 # which won't have any effect.
-
 SRC="$BASH_SOURCE"
 if [ "$SRC" = "" ]; then
   SRC="$0"
@@ -23,7 +22,9 @@ CURDIR="$(pwd)"
 cd "$(dirname "$SRC")"
 unset SRC
 
-./emsdk construct_env "$@"
-. ./emsdk_set_env.sh
+tmpfile=`mktemp` || exit 1
+./emsdk construct_env "$@" $tmpfile
+. $tmpfile
+rm -f $tmpfile
 
 cd "$CURDIR"