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.

Fixes #34053

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>

src/cmd/go/internal/work/exec.go

@@ -1023,7 +1023,7 @@ func (b *Builder) vet(a *Action) error {
 	// dependency tree turn on *more* analysis, as here.
 	// (The unsafeptr check does not write any facts for use by
 	// later vet runs.)
-	if a.Package.Goroot && !VetExplicit {
+	if a.Package.Goroot && !VetExplicit && VetTool == "" {
 		// Note that $GOROOT/src/buildall.bash
 		// does the same for the misc-compile trybots
 		// and should be updated if these flags are