[hal/vulkan] Use Option::insert and avoid an unwrap.

In `wgpu_hal::vulkan::InstanceShared::inspect`, handle
`PhysicalDeviceCapabilities::maintenance_3` more like the way we
handle other extension-provided physical device properties.
Specifically, use `Option::insert` to populate the `Option` and borrow
a mutable reference to its value, rather than calling
`.as_mut().unwrap()`.

This change should have no observable effect on behavior. It simply
replaces a runtime check (`unwrap`) with a statically checked
borrow (`insert`).

wgpu-hal/src/vulkan/adapter.rs

@@ -923,9 +923,10 @@ impl super::InstanceShared {
 
                 let mut builder = vk::PhysicalDeviceProperties2KHR::builder();
                 if supports_maintenance3 {
-                    capabilities.maintenance_3 =
-                        Some(vk::PhysicalDeviceMaintenance3Properties::default());
-                    builder = builder.push_next(capabilities.maintenance_3.as_mut().unwrap());
+                    let next = capabilities
+                        .maintenance_3
+                        .insert(vk::PhysicalDeviceMaintenance3Properties::default());
+                    builder = builder.push_next(next);
                 }
 
                 if supports_descriptor_indexing {