Perforce: P4CONFIG is never set by Qt Creator

The documentation for Qt Creator says to create "p4config.txt" in the top-level directory of the project.
https://doc.qt.io/qtcreator/creator-vcs-perforce.html

However, this file is not used in a default installation of Qt Creator and Perforce!

The Perforce "p4" command-line client will not load any configuration from a file unless the environment variable "P4CONFIG" is set with that specific filename.

On Windows and Linux it is relatively simple for developers to configure this environment variable for GUI applications like Qt Creator.

This is not the case on macOS. It has become increasingly difficult to set an environment variable for GUI applications (launched by Finder/Spotlight etc) on macOS
In Big Sur it now appears to be impossible.

Qt Creator should properly configure the environment for the p4 client, by setting P4CONFIG to the documented value of "p4config.txt" if it not already set in the Qt Creator environment.

This will make the Perforce integration work "out of the box" on Windows, Linux and macOS, while still allowing customisation for users who are using a differently-named p4config file.