Extend PKG_CONFIG_PATH rather than overwriting it

[chrhansk]

Description:

As of now, the setup-python action overwrites the PKG_CONFIG_PATH variable during its execution, which can lead to errors if this variable has already been set to a specific value (which is then no longer available). PKG_CONFIG_PATH works as the normal PATH does (from the man page of pkg-config)

PKG_CONFIG_PATH
  A colon-separated list of high-priority directories where pc(5) files are looked up.

It would therefore be a good idea to instead extend PKG_CONFIG_PATH by adding the new directory to the existing value if one is present in the environment. This will make both existing and new pc files available (We would not want to overwrite PATH either, would we?)

Justification:

I realized this issue after debugging session of a CI pipeline which had a global

env:
  PKG_CONFIG_PATH: '/special/path'

attached to a given workflow. After installing python using setup-python the python dependencies (which included a native library with a pc file) could not be installed any more, since that variable had been overwritten. This is a rather insidious error which is really hard to find.

Are you willing to submit a PR?

Working on a PR.