Make test shard tasks inherit several dimensions from the parent task.

 - Fixes issue where a builder would set os:Ubuntu-22 but its test shards would set os:Linux (b/282142073).
 - Fixes QEMU builders (including Fuchsia ARM64) setting cpu to the dimension of the emulated cpu.
 - Avoids needing to explicitly filter out host_class for Windows ARM64.

Compare https://dart-review.googlesource.com/c/recipes/+/328500
Compare https://dart-review.googlesource.com/c/recipes/+/507080

Change-Id: I399de41b2b6fbbf59fb594369b6286d91ed40813
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/507920
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>