Update cachix/install-nix-action action to v20 (#3)

[![Mend
Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[cachix/install-nix-action](https://togithub.com/cachix/install-nix-action)
| action | major | `v19` -> `v20` |

---

### Release Notes

<details>
<summary>cachix/install-nix-action</summary>

###
[`v20`](https://togithub.com/cachix/install-nix-action/releases/tag/v20):
install-nix-action-v20

[Compare
Source](https://togithub.com/cachix/install-nix-action/compare/v19...v20)

https://discourse.nixos.org/t/nix-2-14-0-released/25900/4Fix support for
Nix 2.14 new profile location:
[https://github.com/NixOS/nix/pull/5226](https://togithub.com/NixOS/nix/pull/5226)

</details>

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined),
Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you
are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the
rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update
again.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

This PR has been generated by [Mend
Renovate](https://www.mend.io/free-developer-tools/renovate/). View
repository job log
[here](https://app.renovatebot.com/dashboard#github/mmlb/each-commit-green).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNC4xNTkuMCIsInVwZGF0ZWRJblZlciI6IjM0LjE1OS4wIn0=-->

Author: mmlb

.github/workflows/ci.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v19
+      - uses: cachix/install-nix-action@v20
       - uses: cachix/cachix-action@v12
         with:
           name: each-commit-green