chore: Nix CI: update setup (#3015)

Now that we're, at least temporarily, relying more on the Nix CI,
replace some old hacks of mine with better solutions people have figured
out in the meantime.

Cachix support could probably be dropped at this point but it doesn't
really hurt.

.github/workflows/nix-ci.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     defaults:
       run:
-        shell: nix -v --experimental-features "nix-command flakes" run .#ciShell -- bash -euxo pipefail {0}
+        shell: nix run .#ciShell -- bash -euxo pipefail {0}
     strategy:
       matrix:
         include:
@@ -29,21 +29,13 @@ jobs:
       fail-fast: false
     name: ${{ matrix.name }}
     env:
-      NIX_BUILD_ARGS: -v --print-build-logs --fallback
+      NIX_BUILD_ARGS: --print-build-logs --fallback
     steps:
       - name: Checkout
         uses: actions/checkout@v3
         with:
           # the default is to use a virtual merge commit between the PR and master: just use the PR
           ref: ${{ github.event.pull_request.head.sha }}
-      - name: Install Nix
-        uses: cachix/install-nix-action@v18
-        with:
-          # https://github.com/NixOS/nix/issues/6572
-          install_url: https://releases.nixos.org/nix/nix-2.7.0/install
-          extra_nix_config: |
-            extra-sandbox-paths = /nix/var/cache/ccache
-            substituters = file://${{ github.workspace }}/nix-store-cache-copy?priority=10&trusted=true https://cache.nixos.org
       - name: Set Up Nix Cache
         uses: actions/cache@v3
         with:
@@ -57,8 +49,13 @@ jobs:
         run: |
           # Nix seems to mutate the cache, so make a copy
           cp -r nix-store-cache nix-store-cache-copy || true
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |
+            extra-sandbox-paths = /nix/var/cache/ccache?
+            substituters = file://${{ github.workspace }}/nix-store-cache-copy?priority=10&trusted=true https://cache.nixos.org  
       - name: Prepare CCache Cache
-        shell: bash -euxo pipefail {0}
         run: |
           sudo mkdir -m0770 -p /nix/var/cache/ccache
           sudo chown -R $USER /nix/var/cache/ccache
@@ -71,7 +68,6 @@ jobs:
           restore-keys: |
             ${{ matrix.name }}-nix-ccache
       - name: Further Set Up CCache Cache
-        shell: bash -euxo pipefail {0}
         run: |
           sudo chown -R root:nixbld /nix/var/cache
           sudo chmod -R 770 /nix/var/cache