Fix for setting multiline outputs

Current way doesn't provide a proper handling.
Creates errors like:
```
Error: Unable to process file command 'output' successfully.
Error: Invalid format 'M	README.md'
```

According to https://github.com/orgs/community/discussions/24952 this should fix the issue.

Author: ChristophShyper

entrypoint.sh

@@ -85,8 +85,12 @@ elif [[ -n ${FILES_CHANGED} ]]; then
 fi
 
 # Finish
-echo "files_changed=${FILES_CHANGED}" >>"$GITHUB_OUTPUT"
-echo "branch_name=${BRANCH}" >>"$GITHUB_OUTPUT"
+{
+  echo "files_changed<<EOF"
+  echo "${FILES_CHANGED}"
+  echo "EOF"
+  echo "branch_name=${BRANCH}"
+} >> "$GITHUB_OUTPUT"
 if [[ ${RET_CODE} != "0" ]]; then
   echo -e "\n[ERROR] Check log for errors."
   exit 1