make.rc: correct test for undefined GOROOT_BOOTSTRAP
The test "if(! ~ $#GOROOT_BOOTSTRAP 1)", to check for the environment
variable GOROOT_BOOTSTRAP being undefined, will not succeed if the
variable is set to the empty string (as the coordinator was doing).
A better test is "if(~ $"GOROOT_BOOTSTRAP '')", which succeeds if
the variable is undefined, or set to an empty list or an empty string.
For #69038
Change-Id: Ic6e6944e0c76461daea206ba9575b863f92f6228
Reviewed-on: https://go-review.googlesource.com/c/go/+/627944
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David du Colombier <0intro@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>