Make REPO in install-pdm.py configurable

[guardian-groot]

Feature Description

Add the possibility in the install script to change the repo to easily install the head version of pdm in an offline environment.

Problem and Solution

Currently you are not able to install the head version of pdm in an offline environment without patching the install script.

Additional Context

In the same context I would remove JSON_URL from the install script as it is nowhere used.

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.

[frostming]

PR welcome.