preserve PKG_CONFIG_PATH instead of overwriting it

[alistair-english]

Hi! I've come across this repo as a depenency for foxglove_compressed_video_transport - thanks for your work on both!

The Problem:
set(ENV{PKG_CONFIG_PATH} ${FFMPEG_PKGCONFIG}) unconditionally overwrites PKG_CONFIG_PATH. Since FFMPEG_PKGCONFIG defaults to an empty string, this blanks out the entire variable during cmake configure.

This works on Ubuntu by accident because ffmpeg's .pc files live in pkg-config's default search path. But it breaks when using Nix or cross-compilation setups.

Proposed fix:

• Prepend FFMPEG_PKGCONFIG to PKG_CONFIG_PATH instead of replacing it, and only when it's non-empty

I also removed a duplicated pkg-config discovery block in CMakeLists.txt

[berndpfrommer]

Thank you for your contribution!
That left-over double block - embarrassing what junk code I commit. How did I not catch this? Time for some LLM to take over. Hopefully, not trained on my code.