fix: error when more than one post-renderer is specified

We now make sure the post-renderer flag can be passed at most once instead of silently taking the last one passed.

Example:

```bash
$ helm template test ...  --post-renderer=true --post-renderer=cat
Error: invalid argument "cat" for "--post-renderer" flag: cannot set post-renderer more than once, given: cat, previous: true
```

Signed-off-by: Yarden Shoham <git@yardenshoham.com>

cmd/helm/flags.go

@@ -24,6 +24,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 	"k8s.io/klog/v2"
@@ -143,6 +144,9 @@ func (p *postRendererString) Set(val string) error {
 	if val == "" {
 		return nil
 	}
+	if p.options.binaryPath != "" && p.options.binaryPath != val {
+		return errors.Errorf("cannot set post-renderer more than once, given: %s, previous: %s", val, p.options.binaryPath)
+	}
 	p.options.binaryPath = val
 	pr, err := postrender.NewExec(p.options.binaryPath, p.options.args...)
 	if err != nil {

cmd/helm/flags_test.go

@@ -20,6 +20,9 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
+	"helm.sh/helm/v4/pkg/action"
 	"helm.sh/helm/v4/pkg/chart"
 	"helm.sh/helm/v4/pkg/release"
 	helmtime "helm.sh/helm/v4/pkg/time"
@@ -93,3 +96,24 @@ func outputFlagCompletionTest(t *testing.T, cmdName string) {
 	}}
 	runTestCmd(t, tests)
 }
+
+func TestPostRendererFlagSetOnce(t *testing.T) {
+	cfg := action.Configuration{}
+	client := action.NewInstall(&cfg)
+	str := postRendererString{
+		options: &postRendererOptions{
+			renderer: &client.PostRenderer,
+		},
+	}
+	// Set the binary once
+	err := str.Set("echo")
+	require.NoError(t, err)
+
+	// Set the binary again to the same value is ok
+	err = str.Set("echo")
+	require.NoError(t, err)
+
+	// Set the binary again to a different value is not ok
+	err = str.Set("cat")
+	require.Error(t, err)
+}