linux kernel determinism: unify timestamp style

Testing showed the linux build is sensitive to /usr/include/ncursesw
unless chrooted (on non-nixos).
On a single chrooted nixos machine, -A linux is binary reproducible.

CC #2281 & @alexanderkjeldaas.

pkgs/os-specific/linux/kernel/generic.nix

···
     autoModules = stdenv.platform.kernelAutoModules;
     arch = stdenv.platform.kernelArch;
 
-    KBUILD_BUILD_TIMESTAMP = 1; # (time_t)1
+    preConfigure = ''
+        buildFlagsArray+=("KBUILD_BUILD_TIMESTAMP=Thu Jan 1 00:00:01 UTC 1970")
+    '';
+
     crossAttrs = let
         cp = stdenv.cross.platform;
       in {

pkgs/os-specific/linux/kernel/manual-config.nix

···
       '';
 
       buildFlags = [
-        "KBUILD_BUILD_TIMESTAMP=1" # (time_t)1
         "KBUILD_BUILD_VERSION=1-NixOS"
         platform.kernelTarget
       ] ++ optional isModular "modules";