internal/tests/deps: skip test if we can't run go command

For golang/go#64942

Change-Id: Idffad578d4bb766dd5c9d8d1a500b8f68598b664
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/553695
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

internal/tests/deps/cmd_pkgsite_deps_test.go

@@ -9,6 +9,8 @@ import (
 	"os/exec"
 	"strings"
 	"testing"
+
+	"golang.org/x/pkgsite/internal/testenv"
 )
 
 // non-test packages are allowed to depend on licensecheck and safehtml, x/ repos, and markdown.
@@ -30,6 +32,8 @@ var additionalAllowedTestModDeps = map[string]bool{
 }
 
 func TestCmdPkgsiteDeps(t *testing.T) {
+	testenv.MustHaveExecPath(t, "go")
+
 	// First, list all dependencies of pkgsite.
 	out, err := exec.Command("go", "list", "-e", "-deps", "golang.org/x/pkgsite/cmd/pkgsite").Output()
 	if err != nil {