[BUG] cmd.run: "- prepend_path:" broken (fails with KeyError exception) on Windows

Description
Due to #56199 I changed some states using cmd.run from

- env:
  - PATH: ...

to

- prepend_path: ...

but that also led to failures on (Windows) minions running Salt Minion 3000.3.

I even get two different error outputs for the same kind of thing (running in a loop):

----------
          ID: something_one
    Function: cmd.run
      Result: False
     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\state.py", line 1981, in call
                  **cdata['kwargs'])
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\loader.py", line 1977, in wrapper
                  return f(*args, **kwargs)
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\states\cmd.py", line 892, in run
                  cret = mod_run_check(cmd_kwargs, onlyif, unless, creates)
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\states\cmd.py", line 366, in mod_run_check
                  cmd = __salt__['cmd.retcode'](unless, ignore_retcode=True, python_shell=True, **cmd_kwargs)
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\modules\cmdmod.py", line 2277, in retcode
                  **kwargs)
                File "c:\salt\bin\lib\site-packages\salt-3000.3-py3.5.egg\salt\modules\cmdmod.py", line 598, in _run
                  run_env['PATH'] = ':'.join((prepend_path, run_env['PATH']))
              KeyError: 'PATH'
     Started: 14:23:09.012359
    Duration: 62.504 ms
     Changes:

and

----------
          ID: something_two
    Function: cmd.run
      Result: False
     Comment: 'PATH'
     Started: 14:24:12.652165
    Duration: 15.624 ms
     Changes:

The reason for this seems to be the code for prepend_path in cmdmod.py, which looks like being Linux/Unix only (https://github.com/saltstack/salt/blob/master/salt/modules/cmdmod.py#L619). It

1. uses a variable name of "PATH", but all Windows systems I looked at use "Path" and the environment variable code seems to be case-sensitive (now?)
2. always uses ":" as path separator, but Windows requires ";"

Setup
Salt Minion 3000.3

Steps to Reproduce the behavior
Execute any cmd.run: state containing - prepend_path: on a Windows Minion running 3000.3

Expected behavior
It should not fail.