builder: fix concurrent map write

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 5ad981640f)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/builder-next/builder.go

@@ -241,7 +241,9 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.
 		}
 
 		defer func() {
+			b.mu.Lock()
 			delete(b.jobs, buildID)
+			b.mu.Unlock()
 		}()
 	}