Revert "core: remove bcache.nix (substituters are taken from nixConfig output of flakes? i suppose?)"

Turns out that's the case for `nix` command usage. Such as `nix run
<flake>#<package>`...

This reverts commit 90ef629.

Author: mrtnvgr

core/bcache.nix

@@ -0,0 +1,15 @@
+{ ... }: {
+  nix.settings = {
+    substituters = [
+      "https://mrtnvgr.cachix.org"
+      "https://nix-community.cachix.org"
+      "https://nix-gaming.cachix.org"
+    ];
+
+    trusted-public-keys = [
+      "mrtnvgr.cachix.org-1:zV5Vv57wyo/NNdiLZg0IMjQSLLL0pkEbfeltQ0F4kL8="
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      "nix-gaming.cachix.org-1:nbjlureqMbRAxR1gJ/f3hxemL9svXaZF/Ees8vCUUs4="
+    ];
+  };
+}

core/default.nix

@@ -5,6 +5,7 @@
     ./packages.nix
     ./fat.nix
     ./overlays.nix
+    ./bcache.nix
     ./home-manager.nix
     ./time-zone.nix
     ./physlock.nix