go/analysis/unitchecker: update go fix, go vet error message

`go fix` and `go vet` have converged, both invoking the same
run function (see go.dev/cl/700795). This CL updates one of the
error messages to correctly indicate which tool was invoked.

Fixes golang/go#77669

Change-Id: I5e7eab4e6c969528e81cbeb3a3dbe16e4cd6cb51
Reviewed-on: https://go-review.googlesource.com/c/tools/+/747060
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: madelinekalil

go/analysis/unitchecker/unitchecker.go

@@ -122,7 +122,7 @@ Usage of %[1]s:
 		os.Exit(0)
 	}
 	if len(args) != 1 || !strings.HasSuffix(args[0], ".cfg") {
-		log.Fatalf(`invoking "go tool vet" directly is unsupported; use "go vet"`)
+		log.Fatalf(`invoking "go tool %[1]s" directly is unsupported; use "go %[1]s"`, progname)
 	}
 	Run(args[0], analyzers)
 }