Properly check for existance of thirdparty dir

Signed-off-by: Jono Yang <jyang@nexb.com>

Author: JonoYang

configure

@@ -55,7 +55,7 @@ CFG_BIN_DIR=$CFG_ROOT_DIR/$VIRTUALENV_DIR/bin
 ################################
 # Thirdparty package locations and index handling
 # Find packages from the local thirdparty directory or from thirdparty.aboutcode.org
-if [ -f "$CFG_ROOT_DIR/thirdparty" ]; then
+if [ -d "$CFG_ROOT_DIR/thirdparty" ]; then
     PIP_EXTRA_ARGS="--find-links $CFG_ROOT_DIR/thirdparty"
 fi
 PIP_EXTRA_ARGS="$PIP_EXTRA_ARGS --find-links https://thirdparty.aboutcode.org/pypi/simple/links.html"