Make sure we dont have hidden options for backend and pipeline compiler (#2123)

move options based on **os.Getenv** into flags

---------
*Sponsored by Kithara Software GmbH*

shared/utils/slices_test.go

@@ -0,0 +1,29 @@
+// Copyright 2023 Woodpecker Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package utils
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMergeSlices(t *testing.T) {
+	resultSS := MergeSlices([]string{}, []string{"a", "b"}, []string{"c"}, nil)
+	assert.EqualValues(t, []string{"a", "b", "c"}, resultSS)
+
+	resultIS := MergeSlices([]int{}, []int{1, 2}, []int{4}, nil)
+	assert.EqualValues(t, []int{1, 2, 4}, resultIS)
+}