[GHA] Fix gen download page script

.github/scripts/update-distro-download-page.sh

@@ -11,10 +11,12 @@ downloads_html="sts4-nightly-${eclipse_version}.html"
 files=`aws s3 cp ${s3_url} . --recursive --exclude "*" --include "spring-tool-suite-4*.zip" --include "spring-tool-suite-4*.dmg" --include "spring-tool-suite-4*.self-extracting.jar" --include "spring-tool-suite-4*.tar.gz" --exclude "*/*" --dryrun`
 rm -f ./${downloads_html}
 echo '<ul>' >> $downloads_html
+s3_url_prefix="s3://${AWS_S3_BUCKET}"
+s3_url_prefix_length=${#s3_url_prefix}
 for file in $files
 do
-  if [[ "$file" =~ ^"s3://${s3_bucket}" ]]; then
-    download_url=${download_url_root}${file:20} #
+  if [[ "$file" =~ ^"${s3_url_prefix}" ]]; then
+    download_url=${download_url_root}${file:$s3_url_prefix_length} #
     filename=${file:${#s3_url}+1}
     echo '  <li><a href="'${download_url}'">'${filename}'</li>' >> $downloads_html
   fi