[buildtools] Use arch-specific GN on linux.

CIPD provides both amd64 and arm64 binaries for GN, download the correct
one. This way we get the correct GN when checking out V8 on an AArch64
linux host.

Change-Id: I39c2d7a0062eb330bccac9bc71068bc02bf6958b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3637798
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80469}

DEPS

@@ -99,7 +99,7 @@ deps = {
   'buildtools/linux64': {
     'packages': [
       {
-        'package': 'gn/gn/linux-amd64',
+        'package': 'gn/gn/linux-${{arch}}',
         'version': Var('gn_version'),
       }
     ],