ci/installer: Refresh workflow infrastructure

The installer workflow had not been touched since 2024 and missed
both the Ubuntu 26.04 runner bump and the move to Determinate Nix as
the common Nix. Bring it in line with `ci.yaml`.

The Go setup step stays: it supports the Gitea release action rather
than the Nix build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/installer.yaml

@@ -7,20 +7,26 @@ on:
 jobs:
   installer:
     name: Build installer
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-26.04
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Go
         uses: https://github.com/actions/setup-go@v4
         with:
           go-version: '>=1.20.1'
-      - uses: cachix/install-nix-action@v27
+      - uses: DeterminateSystems/determinate-nix-action@v3
         with:
-          github_access_token: ${{ secrets.GH_PULL_TOKEN }}
-          extra_nix_config: |
+          # Gitea will supply a token in GITHUB_TOKEN, which this action passes to
+          # Nix (as access-tokens) when downloading from GitHub
+          github-token: ${{ secrets.GH_PULL_TOKEN }}
+          extra-conf: |
             # Make sure we're using sandbox
             sandbox-fallback = false
 
+            # Determinate performance features
+            lazy-trees = true
+            eval-cores = 0
+
             extra-substituters = https://nix-cache.nul.ie
             extra-trusted-public-keys = nix-cache.nul.ie-1:BzH5yMfF4HbzY1C977XzOxoPhEc9Zbu39ftPkUbH+m4=