--pip-args='--constraints https://....' cannot use url based constraints file

[mark-hyde]

Describe the bug
When running:

pipx install foo --pip-args='-c https://example.com/foo/bar/constraints.txt'

...in versions of pipx 1.7.0+ you get an error as follows:

Some possibly relevant errors from pip install:
    ERROR: Could not open requirements file: [Errno 2] No such file or directory: '/home/myusername/https:/example.com/foo/bar/constraints.txt'

...which appears to have prepended my home directory onto the URL where the constraint file would be found.

This works as expected in version 1.6.0 and uses the constraint file from the url location.

My guess would be that it was introduced by #1390