treewide: set intel cpu variant to legacy where newer versions are not supported

According to https://github.com/intel/compute-runtime#legacy-platforms

common/gpu/intel/broadwell/default.nix

@@ -2,6 +2,7 @@
   imports = [ ../. ];
 
   hardware.intelgpu = {
+    computeRuntime = "legacy";
     vaapiDriver = "intel-vaapi-driver";
     enableHybridCodec = true;
   };

common/gpu/intel/coffee-lake/default.nix

@@ -7,5 +7,8 @@
     "i915.enable_guc=2"
   ];
 
-  hardware.intelgpu.vaapiDriver = "intel-media-driver";
+  hardware.intelgpu = {
+    computeRuntime = "legacy";
+    vaapiDriver = "intel-media-driver";
+  };
 }

common/gpu/intel/comet-lake/default.nix

@@ -3,5 +3,8 @@
 
   boot.kernelParams = [ "i915.enable_guc=2" ];
 
-  hardware.intelgpu.vaapiDriver = "intel-media-driver";
+  hardware.intelgpu = {
+    computeRuntime = "legacy";
+    vaapiDriver = "intel-media-driver";
+  };
 }

common/gpu/intel/elkhart-lake/default.nix

@@ -3,5 +3,8 @@
 
   boot.kernelParams = [ "i915.enable_guc=2" ];
 
-  hardware.intelgpu.vaapiDriver = "intel-media-driver";
+  hardware.intelgpu = {
+    computeRuntime = "legacy";
+    vaapiDriver = "intel-media-driver";
+  };
 }

common/gpu/intel/kaby-lake/default.nix

@@ -7,5 +7,8 @@
     "i915.enable_psr=2"
   ];
 
-  hardware.intelgpu.vaapiDriver = "intel-media-driver";
+  hardware.intelgpu = {
+    computeRuntime = "legacy";
+    vaapiDriver = "intel-media-driver";
+  };
 }

common/gpu/intel/skylake/default.nix

@@ -10,6 +10,7 @@
   # NixOS Wiki recommends using the legacy intel-vaapi-driver with the hybrid codec over that one for Skylake.
   # https://wiki.nixos.org/wiki/Accelerated_Video_Playback
   hardware.intelgpu = {
+    computeRuntime = "legacy";
     vaapiDriver = "intel-vaapi-driver";
     enableHybridCodec = true;
   };