Make Bazel-built binaries fully static

[daviddrysdale]

#864 moved binaries built with Cargo to be fully statically linked, based on the musl library. However, this does not affect binaries built with Bazel, which still depend on (e.g.) dynamic linking of libc at runtime:

% ldd ./bazel-clang-bin/oak/server/rust/oak_loader/oak_loader
	linux-vdso.so.1 (0x00007ffddab22000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f0b61eda000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f0b61ecf000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f0b61eae000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f0b61e94000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f0b61cd4000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f0b62bff000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f0b61b8f000)

[daviddrysdale]

#1016 moves the server-side build to be pure-Carg (no Bazel) and so static builds happen by using the --target=x86_64-unknown-linux-musl flag.