Re-add PKG_CONFIG_PATH for armhf image

It doesn't hurt to have this also defined for arm-unknown-linux-gnueabihf
despite apt package installation being disabled by default for this platform. Fixes #1316

docker/Dockerfile.arm-unknown-linux-gnueabihf

@@ -38,6 +38,7 @@ ENV CARGO_TARGET_ARM_UNKNOWN_LINUX_GNUEABIHF_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc
     BINDGEN_EXTRA_CLANG_ARGS_arm_unknown_linux_gnueabihf="--sysroot=$CROSS_SYSROOT" \
     QEMU_LD_PREFIX="$CROSS_SYSROOT" \
     RUST_TEST_THREADS=1 \
+    PKG_CONFIG_PATH="/usr/lib/arm-linux-gnueabihf/pkgconfig/:${PKG_CONFIG_PATH}" \
     CROSS_CMAKE_SYSTEM_NAME=Linux \
     CROSS_CMAKE_SYSTEM_PROCESSOR=arm \
     CROSS_CMAKE_CRT=gnu \