build(nix): add a flake

nix run github:michel/soulseek-rs/master builds the release branch and
/develop the nightly. The lock is not optional: without one nix refuses
a github: reference outright and hides the commit from the build, so
the version stamp read +dirty.

.github/workflows/packaging.yml

@@ -3,6 +3,8 @@ name: Packaging
 on:
   pull_request:
     paths:
+      - 'flake.nix'
+      - 'flake.lock'
       - 'Cargo.lock'
       - 'Cargo.toml'
       - 'packaging/**'
@@ -65,3 +67,21 @@ jobs:
             rpm -ql soulseek-rs | grep -Fx /usr/share/man/man1/soulseek-rs.1.gz
             rpm -ql soulseek-rs | grep -Fx /usr/share/zsh/site-functions/_soulseek-rs
           "
+
+  flake:
+    name: Nix flake
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - uses: cachix/install-nix-action@v31
+
+      - name: Build and run
+        run: |
+          nix build --print-build-logs
+          ./result/bin/soulseek-rs --version | grep -E '\+git[0-9]{12}\.[0-9a-f]{7}$'
+          test -s result/share/man/man1/soulseek-rs.1.gz
+          test -s result/share/fish/vendor_completions.d/soulseek-rs.fish