Use bash's type command to find websockify

Our snap package requires bash to run novnc_proxy, but it doesn't
explicitly set a requirement for the which command to be installed.
Let's therefore use a bash built-in when looking for the websockify
binary.

utils/novnc_proxy

@@ -181,7 +181,7 @@ if [[ -d ${HERE}/websockify ]]; then
 
     echo "Using local websockify at $WEBSOCKIFY"
 else
-    WEBSOCKIFY_FROMSYSTEM=$(which websockify 2>/dev/null)
+    WEBSOCKIFY_FROMSYSTEM=$(type -P websockify 2>/dev/null)
     [ -f $WEBSOCKIFY_FROMSYSTEM ] && WEBSOCKIFY=$WEBSOCKIFY_FROMSYSTEM
 
     if [ ! -f "$WEBSOCKIFY" ]; then