Sandbox libxml2 and switch clang to zstd compression

When trying to execute clang on a very very fresh Debian image, it can't
find libxml2.  Sandbox that too.

While we are here, zstd images extract faster than xz.  Switch over.

Change-Id: Ia196ae49223b488f5eabe28b67e6f274b3072795
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/BUILD b/frc971/BUILD
index 5a357ab..8f749f3 100644
--- a/frc971/BUILD
+++ b/frc971/BUILD
@@ -46,6 +46,7 @@
     ],
     data = [
         "@amd64_debian_sysroot//:sysroot_files",
+        "@clang_amd64_deps//:all",
         "@deb_zlib1g_dev_1_2_11_dfsg_2_amd64_deb_repo//file",
         "@halide_k8//:build_files",
         "@llvm_toolchain//:all-components-x86_64-linux",