Let the backend engine report the current platform (#2688)

if you run woodpecker-agent on windows and connect it to an docker
daemon, there could be two different platforms possible, as you can
switch from linux to windows mode and visa versa


---
*Sponsored by Kithara Software GmbH*

pipeline/backend/common/script_win_test.go

@@ -0,0 +1,58 @@
+// Copyright 2022 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 common
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateScriptWin(t *testing.T) {
+	testdata := []struct {
+		from []string
+		want string
+	}{
+		{
+			from: []string{"echo %PATH%", "go build", "go test"},
+			want: `
+$ErrorActionPreference = 'Stop';
+&cmd /c "mkdir c:\root";
+if ($Env:CI_NETRC_MACHINE) {
+$netrc=[string]::Format("{0}\_netrc",$Env:HOME);
+"machine $Env:CI_NETRC_MACHINE" >> $netrc;
+"login $Env:CI_NETRC_USERNAME" >> $netrc;
+"password $Env:CI_NETRC_PASSWORD" >> $netrc;
+};
+[Environment]::SetEnvironmentVariable("CI_NETRC_PASSWORD",$null);
+[Environment]::SetEnvironmentVariable("CI_SCRIPT",$null);
+
+Write-Output ('+ "echo %PATH%"');
+& echo %PATH%; if ($LASTEXITCODE -ne 0) {exit $LASTEXITCODE}
+
+Write-Output ('+ "go build"');
+& go build; if ($LASTEXITCODE -ne 0) {exit $LASTEXITCODE}
+
+Write-Output ('+ "go test"');
+& go test; if ($LASTEXITCODE -ne 0) {exit $LASTEXITCODE}
+
+`,
+		},
+	}
+	for _, test := range testdata {
+		script := generateScriptWindows(test.from)
+		assert.EqualValues(t, test.want, script, "Want encoded script for %s", test.from)
+	}
+}