Fix CI 32-bit build

A recent change to the Ubuntu Focal CI worker - either a CMake upgrade,
or default installation of the libexpat-dev library - leads to the
32-bit expat library to not be found by CMake.

FindPackage(EXPAT) finds the library via pkg-config, so the fix is
to explicitly include the i386 pkg-config directory in the
PKG_CONFIG_PATH environment variable list.

.github/workflows/osrm-backend.yml

@@ -471,6 +471,7 @@ jobs:
           fi
         elif [[ $TARGET_ARCH == "i686" ]]; then
           source ./scripts/ci/before_install.${TARGET_ARCH}.sh
+          echo "PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig:${PKG_CONFIG_PATH}" >> $GITHUB_ENV
         fi
 
     - name: Prepare build