Support global.json on arm64 as well

arcade uses the runtime section of global.json to decide which
architecture + runtime combination needs to be installed.

With https://github.com/dotnet/arcade/pull/4132 arcade can install
foreign SDKs in separate locations correctly.

This change, suggested by @dougbu, makes arcade always install the
runtime for the local architecture (which means it should work on arm64
and x64) as well as the x86 architecture (skipped on Linux).

This gets us a working SDK/Runtime combo on arm64.

global.json

@@ -5,13 +5,10 @@
   "tools": {
     "dotnet": "5.0.204",
     "runtimes": {
-      "dotnet/x64": [
+      "dotnet": [
         "2.1.28",
         "$(MicrosoftNETCoreAppInternalPackageVersion)"
       ],
-      "dotnet/x86": [
-        "$(MicrosoftNETCoreAppInternalPackageVersion)"
-      ],
       "aspnetcore/x64": [
         "3.1.16"
       ]