add ci via github actions

.github/workflows/ci.yml

@@ -0,0 +1,70 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+env:
+  # Required to make things output color
+  TERM: ansi
+
+jobs:
+  ci:
+    name: CI
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Sync repository
+        uses: actions/checkout@v3
+
+      - name: Uninstall obstructive tools
+        run: rm -rf "$HOME/.rustup" "$HOME/.cargo"
+
+      - name: Install Nix (with flakes and nix-command enabled)
+        uses: cachix/install-nix-action@v23
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+          # Use a version of Nix that doesn't suffer from the following issues:
+          #
+          # * <https://github.com/NixOS/nix/issues/8443>
+          # * <https://github.com/NixOS/nix/issues/9052>
+          install_url: https://releases.nixos.org/nix/nix-2.15.2/install
+
+          # Add the `nix-community` cachix to speed up things that leverage it
+          extra_nix_config: |
+            extra-substituters = https://nix-community.cachix.org
+            extra-trusted-public-keys = nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=
+
+      - name: Configure `nix-direnv`
+        run: |
+          mkdir -p "$HOME/.config/nixpkgs"
+          echo "[(self: super: { nix-direnv = super.nix-direnv.override { enableFlakes = true; }; })]" > "$HOME/.config/nixpkgs/overlays.nix"
+          echo 'source $HOME/.nix-profile/share/nix-direnv/direnvrc' > "$HOME/.direnvrc"
+
+      - name: Install `direnv` and `nix-direnv`
+        run: nix-env -f "<nixpkgs>" -i direnv -i nix-direnv
+
+      - name: Pop/push downloaded crate cache
+        uses: actions/cache@v3
+        with:
+          key: downloaded-crates
+          path: ~/.cargo
+
+      - name: Pop/push compiled crate cache
+        uses: actions/cache@v3
+        with:
+          key: compiled-crates-${{runner.os}}
+          path: target
+
+      # Do this to shorten the logs for the real CI step
+      - name: Populate `/nix/store`
+        run: nix develop --command true
+
+      - name: Perform continuous integration
+        run: |
+          direnv allow
+          direnv exec . engage