dashboard: expect 1 windows-arm64-mini host.
dashboard: expect 1 windows-arm64-mini host.

This will make the dashboard view more consistent.

For golang/go#42604

Change-Id: I90991c9e6c7ba5fc781c6ea1259593048156ca79
Reviewed-on: https://go-review.googlesource.com/c/build/+/333076
Trust: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build