[Navigation Item] Update pin icon button styles (#11972)

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### WHY are these changes introduced?

Fixes Shopify/core-workflows#1095

The pin icon on the navigation item should have a pointer cursor and not
have a background color.
It already has this behaviour for links (which conditionally renders if
you provide a `url` prop), hence the button targeting.

<!--
  Context about the problem that’s being addressed.
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| Before  | After |
| ------------- |:-------------:|
| <img width="810" alt="screenshot"
src="https://screenshot.click/02-16-69404-99751.png"> | <img width="810"
alt="screenshot"
src="https://github.com/Shopify/polaris/assets/42528878/8cf5975e-b0ab-49d2-9a09-68668b277541">
|

### WHAT is this pull request doing?

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Updates the `.SecondaryAction` button's styles

### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#install-dependencies-and-build-workspaces)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

> [!TIP]
> This codesandbox contains this PR's snapshot. To compare with `main`
you can change `@shopify/polaris` version to "latest" in the
`package.json`

Codesandbox:
https://codesandbox.io/p/sandbox/compassionate-wilson-tn6ss3?file=%2FApp.tsx%3A50%2C28
1. Notice that the background of the selected item's secondary action
has no background + has cursor `pointer`

### 🎩 checklist

- [x] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [x] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

.changeset/nervous-forks-know.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': minor
+---
+
+Updated `Navigation.Item` button to have pointer cursor and no background

polaris-react/src/components/Navigation/Navigation.module.css

@@ -684,6 +684,11 @@
   }
 }
 
+.SecondaryAction[type='button'] {
+  cursor: pointer;
+  background: none;
+}
+
 .SecondaryAction {
   display: flex;
   align-items: center;