Otherwise the 2nd find_package(PkgConfig) gets confused:

-- Found PkgConfig: /path/to/pkg-config (found version "0.29")
-- Checking for module 'libavcodec'
--   Found libavcodec, version 58.19.104
-- Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE) (found version "0.29")
CMake Error at /usr/share/cmake-3.12/Modules/FindPkgConfig.cmake:489 (message):
  pkg-config tool not found
Call Stack (most recent call first):
  /usr/share/cmake-3.12/Modules/FindPkgConfig.cmake:602 (_pkg_check_modules_internal)
  CMakeLists.txt:140 (pkg_check_modules)