main.star: fix cipd_platform value for GOHOSTARCH=arm

Handle another way in which cipd_platform is slightly
different from Go's GOHOSTOS and GOHOSTARCH values.

For golang/go#65241.
For golang/go#63698.
For golang/go#63601.

Change-Id: I3caad897b821208939b8b411663ba417c4c21df7
Reviewed-on: https://go-review.googlesource.com/c/build/+/558517
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur