make.bat: autodetect GOROOT_BOOTSTRAP

[FiloSottile]

On Unix we automatically use $(go env GOROOT) as GOROOT_BOOTSTRAP (see #14339). We should do that on Windows as well, where it's harder to set env vars, and for consistency.

Make sure that there are checks in place to prevent $GOROOT_BOOTSTRAP == $GOROOT.

[crvv]

There is a CL for this.
https://go-review.googlesource.com/c/go/+/96455

[gopherbot]

Change https://golang.org/cl/96455 mentions this issue: build: add default GOROOT_BOOTSTRAP in Windows

[gopherbot]

Change https://golang.org/cl/208197 mentions this issue: make.bat: set go env GOROOT as default for GOROOT_BOOTSTRAP