Fix pkg-config search path for cmake when building wasm

[brainos233]

Emscripten's cmake warpper emcmake won't convert environment EM_PKG_CONFIG_PATH to PKG_CONFIG_PATH, which makes cmake's pkg_check_modules() function can't find dependencies in install dir.

For example, libavif will not find libsharpyuv

Details

build-libavif.web-wasm running in /home/aa/Builds/renpy-build/tmp/build/libavif.web-wasm ...
configure: cmake -G Ninja -DCMAKE_FIND_ROOT_PATH=/home/aa/Builds/renpy-build/tmp/install.web-wasm -DCMAKE_SYSTEM_NAME=Emscripten -DCMAKE_SYSTEM_PROCESSOR=generic -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY -DCMAKE_PROJECT_INCLUDE_BEFORE=/home/aa/Builds/renpy-build/tools/cmake_build_variables.cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_BUILD_PARALLEL_LEVEL=12 -DCMAKE_INSTALL_PREFIX=/home/aa/Builds/renpy-build/tmp/install.web-wasm -DAVIF_CODEC_AOM=1 -DAVIF_CODEC_AOM_ENCODE=0 -DBUILD_SHARED_LIBS=0 /home/aa/Builds/renpy-build/tmp/source/libavif -DCMAKE_TOOLCHAIN_FILE=/home/aa/Builds/renpy-build/tmp/cross.web-wasm/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=/home/aa/Builds/renpy-build/tmp/cross.web-wasm/node/22.16.0_64bit/bin/node
-- libavif: Enabling warnings for Clang
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE  
-- Checking for module 'libyuv'
--   Package 'libyuv', required by 'virtual:world', not found
-- libavif: libyuv (1878) found; libyuv-based fast paths enabled.
-- Checking for module 'libsharpyuv'
--   Package 'libsharpyuv', required by 'virtual:world', not found
-- libavif: libsharpyuv not found

This PR's solution is using emmake to warp pkg-config, so environments like EM_PKG_CONFIG_PATH could work correctly in cmake. This is the recommended method by Emscripten.