build: add default GOROOT_BOOTSTRAP in Windows
build: add default GOROOT_BOOTSTRAP in Windows

CL 57753 added support to make.bash and make.rc to
default GOROOT_BOOTSTRAP to 'go env GOROOT'. This
patch does the same in make.bat for Windows.

Updates #18545
Fixes #28641

Change-Id: I9152cc5080ed219b4de5bad0bd12d7725422ee1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/96455
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>

nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go