reattach: don't kill process on failed reconnection

During reattachment, we look to see if the process corresponding to the stored
PID is running. If so, we try to connect to that process. If that fails, we kill
the process under the presumption it's not working, and return
ErrProcessNotFound.

But during reattachment we don't know that the PID we have is still valid. Which
means that the process we're trying to attach to may have exited and a different
process has spawned with the same PID. This results in some unrelated process
getting silently killed.

This impacts Nomad when running the `rawexec` or `exec` task drivers, because
the Nomad agent spawns an "executor" process via go-plugin to control the
workloads, and these executors are left running when Nomad exits. If the
executors die in the meantime (or the host is rebooted), then we can potentially
kill a random process on the host.

Because there's no way for go-plugin to know whether the process is a go-plugin
server without connecting, this kill is never really safe. Remove it.

Ref: hashicorp/nomad#23969
Ref: https://hashicorp.atlassian.net/browse/NET-11233

internal/cmdrunner/cmd_reattach.go

@@ -28,7 +28,6 @@ func ReattachFunc(pid int, addr net.Addr) runner.ReattachFunc {
 		// doesn't actually return an error if it can't find the process.
 		conn, err := net.Dial(addr.Network(), addr.String())
 		if err != nil {
-			p.Kill()
 			return nil, ErrProcessNotFound
 		}
 		conn.Close()