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/commandconn.go

@@ -32,10 +32,6 @@ import (
 	exec "golang.org/x/sys/execabs"
 )
 
-func setPdeathsig(cmd *exec.Cmd) {
-	cmd.SysProcAttr.Pdeathsig = syscall.SIGKILL
-}
-
 func createSession(cmd *exec.Cmd) {
 	// for supporting ssh connection helper with ProxyCommand
 	// https://github.com/docker/cli/issues/1707

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
+}

pkg/upstream/commandconn/pdeathsig_nolinux.go

@@ -0,0 +1,11 @@
+//go:build !linux
+// +build !linux
+
+package commandconn
+
+import (
+	"os/exec"
+)
+
+func setPdeathsig(cmd *exec.Cmd) {
+}