main.star: drop CGO_CFLAGS, CGO_LDFLAGS overrides from linux-arm env

I have some evidence suggesting that setting GOARM=6 is the load-bearing
piece of configuration, not the other two. Take the next step to collect
more data points and test that hypothesis.

For golang/go#65241.

Change-Id: Ia4ecad9da0bc260d9be344e03d48dc83aa01b222
Reviewed-on: https://go-review.googlesource.com/c/build/+/560436
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: dmitshur