[BUG] salt-ssh pre-flight may fail if $TMPDIR is set

[satwell]

Description
salt-ssh pre-flight scripts will fail if $TMPDIR is set and does not exist on the remote host.

salt.client.ssh.run_ssh_pre_flight constructs the remote path for the pre-flight script using tempfile.gettempdir(), which will return the value of $TMPDIR if it's defined. If this directory doesn't exist on the remote host, the script copy will fail.

I ran into this on Linux, but users running salt-ssh from MacOS are also very likely to hit this bug because MacOS sets a per-user $TMPDIR under /var/folders.

Setup
Configure salt-ssh to use any pre-flight script.

Steps to Reproduce the behavior

export TMPDIR="$XDG_RUNTIME_DIR/salt"
mkdir -p "$TMPDIR"
salt-ssh --pre-flight -l debug testhost test.ping

Debug shows an scp to a path constructed from local $TMPDIR:

[DEBUG   ] Terminal Command: scp -o KbdInteractiveAuthentication=no -o PasswordAuthentication=no -o GSSAPIAuthentication=no -o ConnectTimeout=65 -o StrictHostKeyChecking=no -o Port=22 -o IdentityFile=/tmp/salt-master-root/etc/salt/pki/master/ssh/salt-ssh.rsa -o User=root ../states/salt/files/install_salt.sh testhost:/run/user/10551/salt/install_salt.sh

And fails to run at that path because the scp actually failed.

testhost:                                                                                     
    ----------                                                                            
    retcode:                                                                              
        2                                                                                 
    stderr:                                  
        /bin/sh: cannot open /run/user/10551/salt/install_salt.sh: No such file or directory                                                                                        
    stdout:

Expected behavior
salt-ssh should probably always use /tmp as the destination path for the pre-flight script.

Versions Report

salt --versions-report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

Salt Version:
          Salt: 3002.2
 
Dependency Versions:
          cffi: 1.14.3
      cherrypy: Not Installed
      dateutil: 2.8.1
     docker-py: 4.1.0
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 2.11.2
       libgit2: Not Installed
      M2Crypto: Not Installed
          Mako: 1.1.3
       msgpack: 0.6.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: 2.20
      pycrypto: 2.6.1
  pycryptodome: 3.9.7
        pygit2: Not Installed
        Python: 3.8.6 (default, Sep 25 2020, 09:36:53)
  python-gnupg: 0.4.6
        PyYAML: 5.3.1
         PyZMQ: 20.0.0
         smmap: Not Installed
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.3.3
 
System Versions:
          dist: debian testing bullseye
        locale: utf-8
       machine: x86_64
       release: 5.9.0-1-amd64
        system: Linux
       version: Debian GNU/Linux testing bullseye