[release-branch.go1.13] cmd/go/internal/work: fix error while passing custom vet tool

For GOROOT packages, we were adding -unsafeptr=false to prevent unsafe.Pointer
checks. But the flag also got passed to invocations of go vet with a custom
vet tool. To prevent this from happening, we add this flag only when no
tools are passed.

Updates #34053
Fixes #34922

Change-Id: I8bcd637fd8ec423d597fcdab2a0ceedd20786019
Reviewed-on: https://go-review.googlesource.com/c/go/+/200957


Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 902d5aa8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/201237


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>