Fix wrong drone env vars (#1419)

Provide up to date drone compatibility environment variables to each step execution.

closes #1416

Before a step is executed, some environemnt variables are updated.
This ensures, that the updated environment variables are copied to their corresponding `DRONE_` environemt variables.

Side effect is that the `DRONE_` environemnt variables are no longer available in the metadata which should not harm as they are not used inside woodpecker.

pipeline/pipeline.go

@@ -175,6 +175,9 @@ func (r *Runtime) execAll(steps []*backend.Step) <-chan error {
 				return err
 			}
 
+			// add compatibility for drone-ci plugins
+			SetDroneEnviron(step.Environment)
+
 			logger.Debug().
 				Str("Step", step.Name).
 				Msg("Executing")