Use mutable type in writing output

2 jobs for !263 with mutable-podio in 8 seconds (queued for 4 seconds)
latest merge request
Name Stage Failure
failed
compile Build
Uploading artifacts...
WARNING: install/: no matching files
WARNING: build/test_result_*.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
WARNING: build/test_result_*.xml: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1