freebsd: set PKG_CONFIG_PATH in Dockerfile

PKG_CONFIG_PATH is not set in the FreeBSD Dockerfile which means
crates which have an external system dependency cannot find those
dependencies to link against.

Closes cross-rs#1384

docker/Dockerfile.x86_64-unknown-freebsd

@@ -32,6 +32,7 @@ ENV CARGO_TARGET_X86_64_UNKNOWN_FREEBSD_LINKER="$CROSS_TOOLCHAIN_PREFIX"gcc.sh \
     CMAKE_TOOLCHAIN_FILE_x86_64_unknown_freebsd=/opt/toolchain.cmake \
     BINDGEN_EXTRA_CLANG_ARGS_x86_64_unknown_freebsd="--sysroot=$CROSS_SYSROOT" \
     X86_64_UNKNOWN_FREEBSD_OPENSSL_DIR="$CROSS_SYSROOT" \
+    PKG_CONFIG_PATH="${CROSS_SYSROOT}/libdata/pkgconfig/:${PKG_CONFIG_PATH}" \
     CROSS_CMAKE_SYSTEM_NAME=FreeBSD \
     CROSS_CMAKE_SYSTEM_PROCESSOR=amd64 \
     CROSS_CMAKE_CRT=freebsd \