Bump cachix/install-nix-action from 14 to 15 (#91)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 14 to 15.
<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-v15</h2>
<ul>
<li>Enable flakes and the new CLI by default</li>
<li>Fix <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/97">#97</a>: single installation for Act and Docker should be fixed</li>
<li>Fix <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/91">#91</a>: retry official installation as it can sometimes return 400</li>
</ul>
<h2>install-nix-action-v14.1</h2>
<p>github actions disable Spotlight by default now, so we don't have to do it in this action anymore</p>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/cachix/install-nix-action/commit/5f45af07a1f451c75a4ce84e1514200195a1f279"><code>5f45af0</code></a> cleanup</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/489b4a2f78d10342a119552bbdb4cbeddbe02bec"><code>489b4a2</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/105">#105</a> from cachix/resilient-install</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/b93cdea694d1c8ff65df8de50159f7ee185a5394"><code>b93cdea</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/109">#109</a> from cachix/fix-97</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/783053eda638fe1fab03479d925f89f1d41c3755"><code>783053e</code></a> Resilient installation</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/c664ef31269eaab8f4e5c11fd0a4b3b00703d86c"><code>c664ef3</code></a> fix <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/97">#97</a>: install nix config correctly for single-user- mode</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/74ef8ad2cfb373eef5eb62483d242712e1e1ac07"><code>74ef8ad</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/108">#108</a> from cachix/enable-experimental-features-by-default</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/53fb0300a7f3441ce40527e386355609a274f407"><code>53fb030</code></a> enable flakes &amp; nix-command features by default</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/e776d8288cb29f907b00798ed5bdc99ea27b4d63"><code>e776d82</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/cachix/install-nix-action/issues/107">#107</a> from cachix/dependabot/github_actions/actions/checkou...</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/98f3210e45f3cf56c11e228e4ed6b19ccda1cc2a"><code>98f3210</code></a> v14.1</li>
<li><a href="https://github.com/cachix/install-nix-action/commit/8026a8b63f3ebcb2544537e62efc6f694856aab2"><code>8026a8b</code></a> chore(deps): bump actions/checkout from 2.3.5 to 2.4.0</li>
<li>Additional commits viewable in <a href="https://github.com/cachix/install-nix-action/compare/v14...v15">compare view</a></li>
</ul>
</details>
<br />


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</details>

Author: mergify[bot]

.github/workflows/ci.yaml

@@ -40,7 +40,7 @@ jobs:
         push: true
         tags: localhost:5000/tinkerbell/hook-docker:0.0
 
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
 

.github/workflows/push.yaml

@@ -40,7 +40,7 @@ jobs:
         push: true
         tags: quay.io/tinkerbell/hook-docker:0.0
 
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable