CI: Use nixbuild.net

The build takes place on the remote, which takes advantage of Nix's
caching.
The previous workflow used a remote cache but in order to build locally,
all the dependencies needed to be downloaded from the cache everytime.
The dependencies are 462M, downloading took most of the time.

.github/workflows/make-apk.yml

@@ -1,4 +1,4 @@
-name: Make Apk CI
+name: Build-debug-apk
 
 on:
   workflow_dispatch:
@@ -6,29 +6,36 @@ on:
   pull_request:
 
 jobs:
-  Build-Apk:
+  Make-apk:
     runs-on: ubuntu-latest
     steps:
     - name: Install nix
       uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-    - uses: cachix/cachix-action@v10
+    - name: Setup nixbuild.net
+      uses: nixbuild/nixbuild-action@v8
       with:
-        name: julow
-        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-    - name: Checkout Repo
+        nixbuild_ssh_key: ${{ secrets.nixbuild_ssh_key }}
+    - name: Checkout repo
       uses: actions/checkout@v2
     - name: Cache debug certificate
       uses: actions/cache@v2
       with:
         path: _build/debug.keystore
         key: debug-keystore
-    - name: Run nix-shell and Make
-      uses: ZenithalHourlyRate/nix-shell-action@v4
-      with:
-        file: shell.nix
-        script: make
+    # Hopefully cached.
+    - name: Debug certificate
+      run: |
+        [[ -f _build/debug.keystore ]] ||
+        nix-shell ./shell.nix --run 'make debug.keystore'
+    - name: Build
+      run: |
+        nix build --eval-store auto --store ssh-ng://eu.nixbuild.net \
+          -f ./shell.nix debug-apk
+        nix build -f ./shell.nix debug-apk
+        mkdir -p _build
+        cp result/*.apk _build
     - name: Save debug apk
       uses: actions/upload-artifact@v2
       with:

shell.nix

@@ -1,19 +1,35 @@
 { pkgs ? import <nixpkgs> {
   config.android_sdk.accept_license = true;
   config.allowUnfree = true;
-  } }:
+} }:
 
 let
-  jdk = pkgs.openjdk8;
-
   android = pkgs.androidenv.composeAndroidPackages {
     buildToolsVersions = [ "30.0.3" ];
     platformVersions = [ "30" ];
     abiVersions = [ "armeabi-v7a" ];
   };
 
-in
-pkgs.mkShell {
-  buildInputs = [ pkgs.findutils jdk android.androidsdk pkgs.fontforge ];
+  buildInputs =
+    [ pkgs.findutils pkgs.openjdk8 android.androidsdk pkgs.fontforge ];
+
+  # Env variable required by the Makefile
   ANDROID_HOME = "${android.androidsdk}/libexec/android-sdk";
+
+  # Build the debug APK. Exposed as an attribute, used in CI
+  debug-apk = pkgs.stdenv.mkDerivation {
+    name = "unexpected-keyboard-debug";
+    src = ./.;
+    inherit buildInputs ANDROID_HOME;
+    buildPhase = ''
+      make
+    '';
+    installPhase = ''
+      mkdir -p $out
+      mv _build/*.apk $out
+    '';
+  };
+
+in pkgs.mkShell { inherit buildInputs ANDROID_HOME; } // {
+  inherit debug-apk;
 }