ci: migrate Nix CI to official installer and hestia cache

Swap nix-quick-install-action -> nix-installer-action and
cache-nix-action -> Mic92/hestia/action across every Nix workflow, drop
the now-unneeded cache keys, and run devShell tooling through a
defaults.run.shell. Add gc.yml to trim the hestia cache daily.

.github/workflows/test.yml

@@ -6,6 +6,10 @@ concurrency:
   group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
+defaults:
+  run:
+    shell: nix develop --fallback --command bash -e {0}
+
 jobs:
   test:
     runs-on: ubuntu-latest
@@ -27,14 +31,10 @@ jobs:
               - 'integration_test/'
               - 'config-example.yaml'
 
-      - uses: nixbuild/nix-quick-install-action@2c9db80fb984ceb1bcaa77cdda3fdf8cfba92035 # v34
+      - uses: NixOS/nix-installer-action@6b8548fe06acfb0155a50ab5d561accb215764cc # main
         if: steps.changed-files.outputs.files == 'true'
-      - uses: nix-community/cache-nix-action@135667ec418502fa5a3598af6fb9eb733888ce6a # v6.1.3
+      - uses: Mic92/hestia/action@ff07bb902a9968ac0c3d0e51d90a606662a375d8 # main
         if: steps.changed-files.outputs.files == 'true'
-        with:
-          primary-key: nix-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('**/*.nix',
-            '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ runner.arch }}
 
       - name: Run tests
         if: steps.changed-files.outputs.files == 'true'
@@ -44,4 +44,4 @@ jobs:
           # some of the database migration tests.
           LC_ALL: "en_US.UTF-8"
           LC_CTYPE: "en_US.UTF-8"
-        run: nix develop --fallback --command -- gotestsum
+        run: gotestsum