Extend PKG_CONFIG_PATH rather than overwriting it

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.

Issue では、setup-python による PKG_CONFIG_PATH の処理を入口として特定しています。action の環境およびパス書き込みに関するコードと、関連するテストを確認してください。既存の PKG_CONFIG_PATH がある場合の動作を検証し、以前のディレクトリと生成されたディレクトリの両方が引き続き利用可能であることを確認してください。作業はすでに PR #1242 で表されています。