Ensure loaded plugin config is saved to helm server. (#6022)

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### Description of the change

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In [#5339 we inadvertently
switched](https://github.com/vmware-tanzu/kubeapps/pull/5339/files#diff-da1b1406426f28c45f4b63a5ae7547e9c29747e118bf70df6a4694d575710f94R104)
from an [assignment](https://go.dev/ref/spec#Assignment_statements) to a
[short variable
declaration](https://go.dev/ref/spec#Short_variable_declarations):

```diff
-		pluginConfig, err = common.ParsePluginConfig(pluginConfigPath)
+		pluginConfig, err := common.ParsePluginConfig(pluginConfigPath)
```

which meant that, in context, a *new* `pluginConfig` var was created in
that scope, so that the actual `pluginConfig` var from the outside scope
remained empty.

### Benefits

Plugin config works again for the Helm plugin.

### Possible drawbacks

<!-- Describe any known limitations with your change -->

### Applicable issues

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#) -->

- fixes #6020 

### Additional information

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request, mention that information here.-->
I spent some time trying to write a test for this, but I could either
extract the 5 lines to a separate function and test that or test the
`NewServer` function itself. The latter is very difficult because it
instantiates a bunch of network requests, and the former seems redundant
since we already extract the functionality to the
`common.ParsePluginConfig()` function... it's really just a golang
gotcha that is important to be aware of: `a := foo` is equivalent to
`var a atype \n a = foo` and so is always creating a new variable in
that scope.

Signed-off-by: Michael Nelson <minelson@vmware.com>

cmd/kubeapps-apis/plugins/helm/packages/v1alpha1/server.go

@@ -104,8 +104,9 @@ func NewServer(configGetter core.KubernetesConfigGetter, globalPackagingCluster
 	// If no config is provided, we default to the existing values for backwards
 	// compatibility.
 	pluginConfig := common.NewDefaultPluginConfig()
+	var err error
 	if pluginConfigPath != "" {
-		pluginConfig, err := common.ParsePluginConfig(pluginConfigPath)
+		pluginConfig, err = common.ParsePluginConfig(pluginConfigPath)
 		if err != nil {
 			log.Fatalf("%s", err)
 		}