chore(deps): bump cachix/install-nix-action from 20 to 21 (#4040)

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="4b933aa7eb"><code>4b933aa</code></a>
Nix: 2.15.1</li>
<li><a
href="35806937f1"><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="3eb7a24508"><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="840ed7ce9a"><code>840ed7c</code></a>
Document how to pass env vars to modern nix commands</li>
<li><a
href="b2f4229533"><code>b2f4229</code></a>
Fix action to make it work on custom containers;</li>
<li><a
href="e304541747"><code>e304541</code></a>
fix <a
href="https://redirect.github.com/cachix/install-nix-action/issues/170">#170</a></li>
<li><a
href="3988b729f9"><code>3988b72</code></a>
pin Nix to 2.15.0</li>
<li><a
href="763a380571"><code>763a380</code></a>
Bump revision in README</li>
<li><a
href="67e9fd765d"><code>67e9fd7</code></a>
bump revision in readme</li>
<li><a
href="be4cef7b77"><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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.github/workflows/build.yml

@@ -48,7 +48,7 @@ jobs:
       - uses: crazy-max/ghaction-upx@v2
         with:
           install-only: true
-      - uses: cachix/install-nix-action@v20
+      - uses: cachix/install-nix-action@v21
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
       - uses: actions/setup-go@fac708d6674e30b6ba41289acaab6d4b75aa0753 # v4

.github/workflows/release.yml

@@ -101,7 +101,7 @@ jobs:
       - uses: crazy-max/ghaction-upx@v2
         with:
           install-only: true
-      - uses: cachix/install-nix-action@v20
+      - uses: cachix/install-nix-action@v21
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
       - name: dockerhub-login