CMake doesn't find brotli modules when building with CLR_CMAKE_USE_SYSTEM_BROTLI

[tmds]

The command:

./build.sh /p:NoPgoOptimize=true --portablebuild false /p:DotNetBuildFromSource=true /p:DotNetBuildSourceOnly=true /p:DotNetBuildTests=true --cmakeargs -DCLR_CMAKE_USE_SYSTEM_BROTLI=true --cmakeargs -DCLR_CMAKE_USE_SYSTEM_LIBUNWIND=true --cmakeargs -DCLR_CMAKE_USE_SYSTEM_RAPIDJSON=true --cmakeargs -DCLR_CMAKE_USE_SYSTEM_ZLIB=true --runtimeconfiguration Release --librariesConfiguration Debug --subset clr+libs+host+packs+libs.tests

Fails with:

  -- Checking for modules 'brotlidec;brotlienc;brotlicommon'
  --   Package 'brotlidec', required by 'virtual:world', not found
  --   Package 'brotlienc', required by 'virtual:world', not found
  --   Package 'brotlicommon', required by 'virtual:world', not found
  CMake Error at /usr/share/cmake/Modules/FindPkgConfig.cmake:607 (message):
    A required package was not found
  Call Stack (most recent call first):
    /usr/share/cmake/Modules/FindPkgConfig.cmake:829 (_pkg_check_modules_internal)
    /var/home/tmds/repos/runtime/src/native/libs/System.IO.Compression.Native/CMakeLists.txt:11 (pkg_check_modules)

This regressed in #107166.

The system libs configuration isn't exercised in CI which is why the build issue went unnoticed.

@jkoritzinsky can you please take a look?

cc @omajid @MichaelSimons

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