x/build: add longtest builder for darwin/arm64

[cuonglm]

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

cc @golang/release @cherrymui

[ALTree]

Related (for darwin/amd64): #35678.

[gopherbot]

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

[gopherbot]

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