build(deps): bump cachix/install-nix-action from 17 to 18 (#147)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 17 to 18.
<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-v18</h2>
<ul>
<li>Fix nodejs warnings &amp; simplify maintenance</li>
<li>Use python3 when determining number of cores</li>
<li>Collapse installer log messages</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/cachix/install-nix-action/commit/daddc62a2e67d1decb56e028c9fa68344b9b7c2a"><code>daddc62</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/144">#144</a> from cachix/fix-ci</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/8500bf7aa612aa05660b9a22636a967be2dfd1e1"><code>8500bf7</code></a> try to fix act test</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/4024cf0f9d12c1dfdfa2caaa36372ca347dcec12"><code>4024cf0</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/143">#143</a> from ggreif/patch-1</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/3d1155ed72c1fba9d1fa6c46a48ce24218848a4b"><code>3d1155e</code></a> readme: don't perpetuate old versions</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/e17a164a729f3f908f3997516f02ecaba2b9c201"><code>e17a164</code></a> fix <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/140">#140</a>: python -&gt; python3</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/92d36226ca2887d9bfe391bf2d00894d88be3b64"><code>92d3622</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/139">#139</a> from sigprof/collapse-log-output</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/6c5ba55bfdc791cfad61ac72e473340c1c3ac992"><code>6c5ba55</code></a> install-nix.sh: Collapse log messages</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/451e61183802597c1febd6ca3cf18aa163f93a06"><code>451e611</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/134">#134</a> from lovesegfault/composite-action</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/112054fda702ffd8ef0ac620851b57c3c8e36679"><code>112054f</code></a> refactor: replace with a simpler composite action</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/24e801e9d3efe6bac52e0fd3f09d2ab8a815c55f"><code>24e801e</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/123">#123</a> from cachix/dependabot/github_actions/actions/setup-go-3</li>
<li>Additional commits viewable in <a href="https://github.com/cachix/install-nix-action/compare/v17...v18">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@v17
+        uses: cachix/install-nix-action@v18
         with:
           nix_path: nixpkgs=channel:nixos-unstable