speed up kernel build

by having two separate derivations for patching the kernel source tree
and building it, we have to copy said source trees from one store
location to another which takes non-neglible time on spinning rust
(literally minutes on my machine). Replace with a single derivation
that can do more things on one tree in-place

devices/qemu/default.nix

@@ -16,31 +16,15 @@
   overlay = final: prev:
     let inherit (final) stdenvNoCC fetchFromGitHub;
     in {
-      sources = {
-        kernel =
-          let src = fetchFromGitHub {
-                name = "kernel-source";
-                owner = "torvalds";
-                repo = "linux";
-                rev = "3d7cb6b04c3f3115719235cc6866b10326de34cd";  # v5.19
-                hash = "sha256-OVsIRScAnrPleW1vbczRAj5L/SGGht2+GnvZJClMUu4=";
-              };
-          in  stdenvNoCC.mkDerivation {
-            name = "spindled-kernel-tree";
-            inherit src;
-            phases = [
-              "unpackPhase"
-              "patchScripts" "installPhase"
-            ];
-
-            patchScripts = ''
-              patchShebangs scripts/
-            '';
-            installPhase = ''
-              mkdir -p $out
-              cp -a . $out
-            '';
-          };
+      kernel = prev.kernel.override {
+        # using fetchurl not fetchzip because it doesn't unpack, and
+        # copying 6GB of data from one store location to another
+        # takes an absolute bloody age
+        src = final.fetchurl {
+          name = "linux.tar.gz";
+          url = "https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.19.16.tar.gz";
+          hash = "sha256-m4NeoEsCEK0HSIKTZ6zYTgk1fD3W0PSOMXN6fyHpkP8=";
+        };
       };
     };