Add CI workflow using Nix

.github/workflows/build-nix-on-ubuntu.yml

@@ -0,0 +1,27 @@
+name: CMake Build (Nix on Ubuntu x86-64)
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v12
+      with:
+        nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/a127deeb889b111138f5152e54681577706ab741.tar.gz
+        # ^ this is also specified in the expression file, but I think using any other value here may cause superfluous downloads from the binary cache
+    - name: Grab dependencies and build # and move output to CWD
+      run: |
+        nix-build ./ci-build-drv.nix
+        cp -r "$(realpath "$PWD/result")/bin" ${{runner.workspace}}
+    - uses: actions/upload-artifact@v2
+      with:
+        name: melonDS
+        path: ${{runner.workspace}}/bin

ci-build-drv.nix

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+# mostly copied from nixpkgs: https://github.com/NixOS/nixpkgs/blob/a127deeb889b111138f5152e54681577706ab741/pkgs/misc/emulators/melonDS/default.nix
+# grab updates from there (at master, maybe bump pinned nixpkgs too), though I don't expect any huge changes
+
+{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/a127deeb889b111138f5152e54681577706ab741.tar.gz") {}
+, mkDerivation ? pkgs.stdenv.mkDerivation
+, cmake ? pkgs.cmake
+, pkg-config ? pkgs.pkg-config
+, SDL2 ? pkgs.SDL2
+, qtbase ? pkgs.qt5.qtbase
+, epoxy ? pkgs.epoxy
+, libarchive ? pkgs.libarchive
+, libpcap ? pkgs.libpcap
+, libslirp ? pkgs.libslirp
+, pcre ? pkgs.pcre
+, wrapGAppsHook ? pkgs.wrapGAppsHook
+}:
+
+mkDerivation {
+  name = "melonDS-git-ci";
+
+  src = builtins.path { path = ./.; name = "melonDS"; }; # use the directory this file is in (as opposed to downloading a source tarball)
+
+  nativeBuildInputs = [ cmake pkg-config wrapGAppsHook ];
+  buildInputs = [
+    SDL2
+    qtbase
+    epoxy
+    libarchive
+    libpcap
+    libslirp
+    pcre
+  ];
+
+  cmakeFlags = [ "-UUNIX_PORTABLE" ];
+}