[Bug]: cmd.run environment inconsistent between local and remote execution

What happened?

cmd.run uses a different default shell depending on how the job is started:

# salt-call cmd.run 'echo $0 - $SHELL'
local:
    /bin/bash - /bin/bash
# salt $HOSTNAME 'echo $0 - $SHELL'
salt.example.com:
    /bin/sh - 

The grain value is also different:

# salt-call grains.get shell
local:
    /bin/bash
# salt $HOSTNAME grains.get shell
salt.example.com:
    /bin/sh

Presumably it is using the current shell of the calling process, and not the "system's default shell" as documented:

shell (str) -- Specify an alternate shell. Defaults to the system's default shell.

Type of salt install

Official deb

Major version

3006.x

What supported OS are you seeing the problem on? Can select multiple. (If bug appears on an unsupported OS, please open a GitHub Discussion instead)

ubuntu-24.04

salt --versions-report output

Salt Version:
          Salt: 3006.27

Python Version:
        Python: 3.11.15 (main, Jun 29 2026, 22:21:49) [GCC 11.2.0]

Dependency Versions:
          cffi: 2.0.0
      cherrypy: 18.10.0
  cryptography: 47.0.0
      dateutil: 2.9.0.post0
     docker-py: Not Installed
         gitdb: 4.0.12
     gitpython: 3.1.50
        Jinja2: 3.1.6
       libgit2: 1.9.6
  looseversion: 1.3.0
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.1.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 24.0
     pycparser: 3.00
      pycrypto: Not Installed
  pycryptodome: 3.23.0
        pygit2: 1.20.0
  python-gnupg: 0.5.6
        PyYAML: 6.0.3
         PyZMQ: 27.1.0
        relenv: 0.22.16
         smmap: 5.0.2
       timelib: 0.3.0
       Tornado: 6.5.5
           ZMQ: 4.3.5

System Versions:
          dist: ubuntu 24.04.4 noble
        locale: utf-8
       machine: x86_64
       release: 6.8.0-137-generic
        system: Linux
       version: Ubuntu 24.04.4 noble

Start by reproducing the documented cmd.run and grains.get shell comparisons with salt-call and remote salt on Ubuntu 24.04. Trace how shell defaults are selected for local and remote execution, and verify that both use the documented system default consistently with regression coverage for the observed difference.