.ci: Add instantiate-all.nix

This is to be used with `nix-env` in a CI situation to list everything
from release.nix.

.ci/instantiate-all.nix

@@ -0,0 +1,63 @@
+# This file is used to flattenize release.nix for `nix-env` use.
+# In turn, this is used by the CI steps for validating the evaluation is still
+# working fine, and to get a list of diffs for the changes.
+let
+  # Evaluate release.nix
+  release = import ../release.nix {
+    systems = [ "aarch64-linux" "x86_64-linux" ];
+  };
+
+  # Get "a" Nixpkgs for its lib.
+  pkgs = import <nixpkgs> {};
+  inherit (pkgs) lib;
+  # And import what we need in scope.
+  inherit (lib.attrsets) getAttrFromPath nameValuePair mapAttrs' mapAttrsToList;
+  inherit (lib.strings) splitString;
+  inherit (lib.lists) flatten;
+
+  # Given a path, returns the attrset at that path in release,
+  # with the attribute names prefixed with path.
+  dig = dig' release;
+
+  # Given a path, returns the attrset at that path in attrset,
+  # with the attribute names prefixed with path.
+  dig' = attrset: path:
+    mapAttrs' (name: value: nameValuePair "${path}.${name}" value)
+    (getAttrFromPath (splitString "." path) attrset)
+  ;
+
+  # Flattened attrset of device builds.
+  devices =
+    let
+      # Listifies devices in a list of lists of nameValuePairs.
+      # This will be flattened and re-hydrated in a shallow attrset.
+      devicesList =
+        mapAttrsToList
+        (deviceName: list: mapAttrsToList (platformName: value: {
+            name = "device.${deviceName}.${platformName}";
+            value = value;
+          }) list)
+        release.device
+      ;
+    in
+      builtins.listToAttrs (flatten devicesList)
+    ;
+
+  flattened =
+    release //
+    devices //
+    # We could try and do something smart to unwrap two levels of attrsets
+    # automatically, but by stating we want those paths we are ensuring that
+    # they are still present in the attrsets.
+    (dig "overlay.aarch64-linux.aarch64-linux") //
+    (dig "overlay.x86_64-linux.aarch64-linux-cross") //
+    (dig "overlay.x86_64-linux.armv7l-linux-cross") //
+    (dig "overlay.x86_64-linux.x86_64-linux")
+  ;
+
+  # Escape the name so `nix-env` will show it.
+  escapeName = builtins.replaceStrings ["."] ["++"];
+in
+  mapAttrs'
+  (name: value: nameValuePair (escapeName name) value)
+  flattened