Set explicit GOARCH

[dpogorzelski]

Building on M1 macs:

GOOS=linux go build hello.go

will result in:

signal 11 received by tid 2, errno 0, code 1
   fault address 0x0

Set explicit GOARCH to avoid

[eyberg]

@dpogorzelski i'm ok w/this change although it might help to provide some context:

1. this is necessary on m1s but not intel macs cause it's arm

2. we actually do support native arm as well if running with a nanos arm build

so maybe it makes sense to add these notes?