cmd/dist: builds amd64 toolchain on macOS arm64 system

[rsc]

p1% env|grep GO
GOPATH=/Users/rsc
GOROOT=/Users/rsc/go
p1% ./make.bash
Building Go cmd/dist using /Users/rsc/sdk/go1.17. (go1.17 darwin/arm64) <<<<
Building Go toolchain1 using /Users/rsc/sdk/go1.17.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
Building Go toolchain2 using go_bootstrap and Go toolchain1.
Building Go toolchain3 using go_bootstrap and Go toolchain2.
Building packages and commands for darwin/amd64.  <<<<
---
Installed Go for darwin/amd64 in /Users/rsc/go.
Installed commands in /Users/rsc/go/bin
p1% 

Note the two marked lines: the bootstrap toolchain is ARM64 but somehow cmd/dist has reverted to x86-64.
I know this is caused by running under some x86 process, like my shell or terminal or whatever.
It doesn't matter: this is incredibly confusing and we should stop it if we can. And we can.
I will send a CL.

[gopherbot]

Change https://golang.org/cl/378894 mentions this issue: cmd/dist: avoid lapsing into x86 builds on ARM64 Macs