fix: use x-platform code for pdeathsig

This might cause the macosx build not to fail, I hope.

See https://github.com/docker/cli/tree/v20.10.10/cli/connhelper/commandconn

pkg/upstream/commandconn/pdeathsig_linux.go

@@ -0,0 +1,10 @@
+package commandconn
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func setPdeathsig(cmd *exec.Cmd) {
+	cmd.SysProcAttr.Pdeathsig = syscall.SIGKILL
+}