Support for output redirect + os-independent environment variable substitution in `cmd`

[schwaerz]

We are trying to use pdm as a wrapper to Conan projects.

What we are doing is:

• Install Conan and other tools using python packages
• Define aliases for most commonly used commands (as an alternative to dedicated scripts)
• Some of the commands need both environment variable subsitution as well as redirecting the output into a file

Currently I have the choice between cmd and shell. AFAIK only with shell I can redirect the output. However I cannot use shell as I'd have a different command line wrt to using environment variables %VAR% vs ${VAR}, depending on the OS.

Describe the solution you'd like

• Possbility to redirect the output as part of cmd into a file

[schwaerz]

We found following workaround:

my_script.shell = "bash -c \"echo ${VAR}\""