make.bash, make.rc: fix GOROOT detection when GOEXPERIMENT is set

make.bash, make.rc: fix GOROOT detection when GOEXPERIMENT is set

We need to clear GOEXPERIMENT any time we are invoking a bootstrap
toolchain. One line missed the clearing of GOEXPERIMENT.

There were three different lines using different syntaxes and subtly
different sets of variables being cleared, so hoist them into a function
so it's all in one place.

Also quote $GOROOT_BOOTSTRAP consistently.

Change-Id: I6c5a5d70c694c24705bbc61298b28ae906c0cf6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/456635
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>