Use PYTHONTRACEMALLOC=25 as envvar to enable tracemalloc on start
And then:
```
export SWH_CONFIG_FILENAME=/etc/softwareheritage/loader_git.yml
export PYTHONTRACEMALLOC=25
swh --log-level DEBUG loader run git <origin-url>
```
Don't forget the log-level flag otherwise, you won't see anything.
Related to T3625