Revert "ssh/tailssh: change to user directory when running login/command"

This reverts commit dc5bc32d8fbacd7e1dd34d138de542d401a11ebd.

It broke tests. (sadly, ones which we have disabled on CI, but go test
./ssh/tailssh broke)

ssh/tailssh/incubator.go

@@ -113,7 +113,6 @@ func (ss *sshSession) newIncubatorCommand() (cmd *exec.Cmd) {
 		"--remote-ip=" + ci.src.Addr().String(),
 		"--has-tty=false", // updated in-place by startWithPTY
 		"--tty-name=",     // updated in-place by startWithPTY
-		"--pwd=" + ss.conn.localUser.HomeDir,
 	}
 
 	if isSFTP {
@@ -178,7 +177,6 @@ type incubatorArgs struct {
 	isShell      bool
 	loginCmdPath string
 	cmdArgs      []string
-	pwd          string
 }
 
 func parseIncubatorArgs(args []string) (a incubatorArgs) {
@@ -195,7 +193,6 @@ func parseIncubatorArgs(args []string) (a incubatorArgs) {
 	flags.BoolVar(&a.isShell, "shell", false, "is launching a shell (with no cmds)")
 	flags.BoolVar(&a.isSFTP, "sftp", false, "run sftp server (cmd is ignored)")
 	flags.StringVar(&a.loginCmdPath, "login-cmd", "", "the path to `login` cmd")
-	flags.StringVar(&a.pwd, "pwd", "/", "process initial working directory, if possible. else / is used")
 	flags.Parse(args)
 	a.cmdArgs = flags.Args()
 	return a
@@ -282,12 +279,6 @@ func beIncubator(args []string) error {
 	cmd.Stderr = os.Stderr
 	cmd.Env = os.Environ()
 
-	if _, err := os.Stat(ia.pwd); err != nil && os.IsNotExist(err) {
-		cmd.Dir = "/"
-	} else {
-		cmd.Dir = ia.pwd
-	}
-
 	if ia.hasTTY {
 		// If we were launched with a tty then we should
 		// mark that as the ctty of the child. However,
@@ -437,7 +428,16 @@ func (ss *sshSession) launchProcess() error {
 	ss.cmd = ss.newIncubatorCommand()
 
 	cmd := ss.cmd
-
+	homeDir := ss.conn.localUser.HomeDir
+	if _, err := os.Stat(homeDir); err == nil {
+		cmd.Dir = homeDir
+	} else if os.IsNotExist(err) {
+		// If the home directory doesn't exist, we can't chdir to it.
+		// Instead, we'll chdir to the root directory.
+		cmd.Dir = "/"
+	} else {
+		return err
+	}
 	cmd.Env = envForUser(ss.conn.localUser)
 	for _, kv := range ss.Environ() {
 		if acceptEnvPair(kv) {