make.bash: avoid warning when bootstrap doesn't support GOOS/GOARCH
make.bash: avoid warning when bootstrap doesn't support GOOS/GOARCH

Currently, if make.bash is run with a GOOS or GOARCH that the
bootstrap toolchain doesn't support, it will print an ominous but
harmless warning like:

  2020/09/21 09:05:27 unsupported GOARCH arm64

This comes from the invocation of "go version" to get the exact
bootstrap toolchain version.

Since the GOOS and GOARCH don't matter for this purpose, this CL
simply clears them on the invocation of the bootstrap toolchain's "go
version".

Fixes #41525.

Change-Id: I17d44eaafed9999b9fa7dcb9fb100b5fd5e554d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/256297
Trust: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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