build(deps): bump cachix/install-nix-action from 20 to 21 (#170)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 20 to 21.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/cachix/install-nix-action/releases">cachix/install-nix-action's releases</a>.</em></p>
<blockquote>
<h2>install-nix-action-v21</h2>
<ul>
<li>pin Nix to 2.15.1 (recent releases broke too many things)</li>
<li>fix the action to work on custom containers</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/cachix/install-nix-action/commit/4b933aa7ebcc94a6174cf1364864e957b4910265"><code>4b933aa</code></a> Nix: 2.15.1</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/35806937f12c927d9c7223431c7261e2eb2a33ef"><code>3580693</code></a> Merge pull request <a href="https://redirect.github.com/cachix/install-nix-action/issues/179">#179</a> from joergdw/fix-action-path</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/3eb7a24508f99341eb0bc0d38e72b87d370f51a3"><code>3eb7a24</code></a> Merge pull request <a href="https://redirect.github.com/cachix/install-nix-action/issues/178">#178</a> from cachix/docs/149</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/840ed7ce9ada3a8d0ff69bc6166412e5a51f3b60"><code>840ed7c</code></a> Document how to pass env vars to modern nix commands</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/b2f42295335195f8a65f0786d699df4accea6cc4"><code>b2f4229</code></a> Fix action to make it work on custom containers;</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/e304541747dc88aedd826d105fd054d7b2061513"><code>e304541</code></a> fix <a href="https://redirect.github.com/cachix/install-nix-action/issues/170">#170</a></li>
<li><a href="https://github.com/cachix/install-nix-action/commit/3988b729f99a669755c5ef20aea9794783503d9d"><code>3988b72</code></a> pin Nix to 2.15.0</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/763a380571be842cf54ef33c086be2fa7e8dbad9"><code>763a380</code></a> Bump revision in README</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/67e9fd765dbe63fabe0ce2bd72f56f9a417696a0"><code>67e9fd7</code></a> bump revision in readme</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/be4cef7b776998e97233d6e0b84c538eb8122d76"><code>be4cef7</code></a> Merge pull request <a href="https://redirect.github.com/cachix/install-nix-action/issues/166">#166</a> from l0b0/refactor/linting</li>
<li>Additional commits viewable in <a href="https://github.com/cachix/install-nix-action/compare/v20...v21">compare view</a></li>
</ul>
</details>
<br />


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</details>

Author: mergify[bot]

.github/workflows/ci.yaml

@@ -39,7 +39,7 @@ jobs:
           echo ::set-output name=short::$(git rev-parse --short HEAD)
 
       - name: Install nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           nix_path: nixpkgs=channel:nixos-unstable