Auto merge of servo#28902 - servo:dependabot/cargo/spin-0.9.4, r=jdm

Bump spin from 0.9.2 to 0.9.4

Bumps [spin](https://github.com/mvdnes/spin-rs) from 0.9.2 to 0.9.4.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/mvdnes/spin-rs/blob/master/CHANGELOG.md">spin's changelog</a>.</em></p>
<blockquote>
<h1>[0.9.4] - 2022-07-14</h1>
<h3>Fixed</h3>
<ul>
<li>Fixed unsoundness in <code>RwLock</code> on reader overflow</li>
<li>Relaxed <code>Send</code>/<code>Sync</code> bounds for <code>SpinMutex</code> and <code>TicketMutex</code> (doesn't affect <code>Mutex</code> itself)</li>
</ul>
<h1>[0.9.3] - 2022-04-17</h1>
<h3>Added</h3>
<ul>
<li>Implemented <code>Default</code> for <code>Once</code></li>
<li><code>Once::try_call_once</code></li>
</ul>
<h3>Fixed</h3>
<ul>
<li>Fixed bug that caused <code>Once::call_once</code> to incorrectly fail</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li>See full diff in <a href="https://github.com/mvdnes/spin-rs/commits">compare view</a></li>
</ul>
</details>
<br />

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</details>

Author: bors-servo