local backend: fix steps having logs form other steps (#5582)

## problem

if steps where started concurrent, the stdout pipeline reader war overwritten and you randomly got the wrong command stream 
from a step.

## change

where we have possible race conditions, we now use thread save types
e.g. store the command struct and the output reader in sync.Map

also a lot of tests where added

pipeline/backend/local/const.go

@@ -15,7 +15,6 @@
 package local
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -30,11 +29,6 @@ var notAllowedEnvVarOverwrites = []string{
 	"CI_WORKSPACE",
 }
 
-var (
-	ErrUnsupportedStepType   = errors.New("unsupported step type")
-	ErrWorkflowStateNotFound = errors.New("workflow state not found")
-)
-
 const netrcFile = `
 machine %s
 login %s