cmd/go/testdata/script: clear path in gotoolchain_issue66175
The gotoolchain_issue66175 script test would fail if a binary named
go1.21 or go1.22 was present in the system PATH. Clear the path at the
beginning of the test to make it independent of the system path.
Fixes #67277
Change-Id: I6d9133aee5b20ab116e07b659e8ecf3eb8add4ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/584258
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>