Fix path quoting bug introduced in #220 (#227)

It seems that some variables were mistakenly left unquoted, leading to paths containing spaces being treated incorrectly.

For example, the directory `/home/person/foo bar/test/` would cause `CURDIR=$(pwd)` to be expanded to `CURDIR=/home/person/foo bar/test/`. This means that the shell would attempt to run the command `bar/test/` with the environment variable `CURDIR` set to `/home/person/foo`. This is due to [word splitting](https://www.gnu.org/software/bash/manual/html_node/Word-Splitting.html). (TL;DR the result of shell expansions should probably be quoted 99% of the time)

emsdk_env.sh

@@ -19,11 +19,11 @@ SRC="$BASH_SOURCE"
 if [ "$SRC" = "" ]; then
   SRC="$0"
 fi
-CURDIR=$(pwd)
-cd $(dirname "$SRC")
+CURDIR="$(pwd)"
+cd "$(dirname "$SRC")"
 unset SRC
 
 ./emsdk construct_env "$@"
 . ./emsdk_set_env.sh
 
-cd $CURDIR
+cd "$CURDIR"