flake.nix: add static cross-compiled builds

The nix invocations come from the garage .nix files

compile.nix

@@ -0,0 +1,44 @@
+{ system, target ? null, pkgsSrc, cargo2nixOverlay, compiler ? "rustc",
+  release ? false, }:
+
+let
+  pkgs = if target != null then
+    import pkgsSrc {
+      inherit system;
+      crossSystem = {
+        config = target;
+        isStatic = true;
+      };
+      overlays = [ cargo2nixOverlay ];
+    }
+  else
+    import pkgsSrc {
+      inherit system;
+      overlays = [ cargo2nixOverlay ];
+    };
+
+  toolchainOptions = {
+    rustVersion = "1.75.0";
+  };
+
+  packageFun = import ./Cargo.nix;
+
+  codegenOpts = {
+    "armv6l-unknown-linux-musleabihf" = [
+      "target-feature=+crt-static"
+      "link-arg=-static"
+    ]; # compile as dynamic with static-pie
+    "x86_64-unknown-linux-musl" =
+      [ "target-feature=+crt-static" "link-arg=-static-pie" ];
+  };
+
+  # NixOS and Rust/Cargo triples do not match for ARM, fix it here.
+  rustTarget = if target == "armv6l-unknown-linux-musleabihf" then
+    "arm-unknown-linux-musleabihf"
+  else
+    target;
+
+in pkgs.rustBuilder.makePackageSet ({
+  inherit release packageFun codegenOpts;
+  target = rustTarget;
+} // toolchainOptions)

flake.nix

@@ -10,25 +10,36 @@
   };
 
   outputs = { self, nixpkgs, cargo2nix, flake-utils }:
+    let
+      compile = import ./compile.nix;
+    in
     flake-utils.lib.eachDefaultSystem (system:
       let
-        pkgs = import nixpkgs {
-          inherit system;
-          overlays = [cargo2nix.overlays.default];
-        };
-
-        rustPkgs = pkgs.rustBuilder.makePackageSet {
-          rustVersion = "1.75.0";
-          packageFun = import ./Cargo.nix;
-        };
-
-      in rec {
-        packages = {
-          restic-alarm = (rustPkgs.workspace.restic-alarm {});
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      rec {
+        packages =
+          let
+            packageFor = target: (compile {
+              inherit system target;
+              pkgsSrc = nixpkgs;
+              cargo2nixOverlay = cargo2nix.overlays.default;
+              release = true;
+            }).workspace.restic-alarm { compileMode = "build"; };
+          in
+          {
+            # default = native release build
             default = packages.restic-alarm;
+            restic-alarm = packageFor null;
+            # other = cross-compiled, statically-linked builds
+            amd64 = packageFor "x86_64-unknown-linux-musl";
+            i386 = packageFor "i686-unknown-linux-musl";
+            arm64 = packageFor "aarch64-unknown-linux-musl";
+            arm = packageFor "armv6l-unknown-linux-musleabihf";
           };
       }
-    ) // rec {
+    )
+    // rec {
       nixosModules.restic-alarm = { config, lib, pkgs, ... }:
         with lib;
         let