fix(gatsby-source-filesystem): fix race condition when using publicURL field (#28176) (#28209)

Co-authored-by: cws <[email protected]>
(cherry picked from commit 1abf65c)

Co-authored-by: Joost Jansky <[email protected]>

Author: vladar

packages/gatsby-source-filesystem/src/extend-file-node.js

@@ -30,7 +30,7 @@ module.exports = ({
         )
 
         if (!fs.existsSync(publicPath)) {
-          fs.copy(
+          fs.copySync(
             details.absolutePath,
             publicPath,
             { dereference: true },