fix a typo in the cygwin tmpdir support

etc/kaleidoscope-builder.conf

@@ -155,7 +155,7 @@ elif [ "${uname_O}" = "Cygwin" ]; then
     # Windows Arduino binaries can understand.
     : "${ARDUINO_PATH:=/cygdrive/c/Program\ Files\ (x86)/Arduino}"
     : "${ARDUINO_LOCAL_LIB_PATH:=/cygdrive/c/Users/${USER}/Arduino}"
-    TMPDIR="${ARDUINO_LOCAL_LIB_PATH:-/cygdrive/c/Users/${USER}/AppData/Local/Temp}"
+    : "${TMPDIR:=/cygdrive/c/Users/${USER}/AppData/Local/Temp}"
 
     # We need to prevent Windows executables from being passed parameters that are absolute paths, since they won't
     # be interpretable when of the form /cygdrive/c/foo. To work around this, we set the common path root variables