race in bootstrap phase: sourceRef of management-sylva-units, GitRepository lookup done before it exists

https://gitlab.com/sylva-projects/sylva-core/-/jobs/7607144535#L327:

2024/08/18 12:52:49.892164 HelmRelease/sylva-units state changed: InstallFailed - Helm install failed for release sylva-units-preview/sylva-units with chart sylva-units@0.0.0-git+4798e1365b92.1: execution error at (sylva-units/templates/units.yaml:53:12): repo_name is empty for unit management-sylva-units: %!!(MISSING)s(<nil>)

This seems to happen because the lookup for sourceRef in management-sylva-units (introduce in !2694 (merged)) is done before the GitRepository is created.

/cc @feleouet