Update cachix/install-nix-action action to v29 (#143)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[cachix/install-nix-action](https://redirect.github.com/cachix/install-nix-action)
| action | major | `v27` -> `v29` |

---

### Release Notes

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

###
[`v29`](https://redirect.github.com/cachix/install-nix-action/releases/tag/v29)

[Compare
Source](https://redirect.github.com/cachix/install-nix-action/compare/v27...v29)

Bumps Nix to 2.24.8 to fix CVE-2024-47174

</details>

---

### Configuration

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

🚦 **Automerge**: Enabled.

♻ **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 was generated by [Mend Renovate](https://mend.io/renovate/).
View the [repository job
log](https://developer.mend.io/github/bobvanderlinden/nixpkgs-ruby).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzOC45Ny4wIiwidXBkYXRlZEluVmVyIjoiMzguOTcuMCIsInRhcmdldEJyYW5jaCI6Im1hc3RlciIsImxhYmVscyI6W119-->

.github/workflows/check.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: Install Nix
-        uses: cachix/install-nix-action@v27
+        uses: cachix/install-nix-action@v29
       - uses: cachix/cachix-action@v15
         with:
           name: nixpkgs-ruby

.github/workflows/update.yml

@@ -16,7 +16,7 @@ jobs:
       with:
           ref: master
     - name: Install Nix
-      uses: cachix/install-nix-action@v27
+      uses: cachix/install-nix-action@v29
     - run: nix run .#update
       env:
         GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}