Golang x/build: add longtest builder for darwin/arm64

See for example #49032, which I think is not caught due to missing longtest builder for darwin/arm64.

cc @golang/release @cherrymui

Comment From: ALTree

Related (for darwin/amd64): #35678.

Comment From: gopherbot

Change https://go.dev/cl/474137 mentions this issue: cmd/go: don't run slow tests on non-longtest builders

Comment From: gopherbot

Change https://go.dev/cl/474580 mentions this issue: [release-branch.go1.20] cmd/go: avoid running slow tests on non-longtest builders

Comment From: gopherbot

Change https://go.dev/cl/474581 mentions this issue: [release-branch.go1.19] cmd/go: avoid running slow tests on non-longtest builders

Comment From: dmitshur

This is done. Closing in favor of the newer issue #73488.