Makefile: build libgit-rs and libgit-sys serially

Makefile: build libgit-rs and libgit-sys serially

"make -JN" with INCLUDE_LIBGIT_RS enabled causes cargo lock warnings
and can trigger ld errors during the build.

The build errors are caused by two inner "make" invocations getting
triggered concurrently: once inside of libgit-sys and another inside of
libgit-rs.

Make libgit-rs depend on libgit-sys so that "make" prevents them
from running concurrently. Apply the same logic to the test invocations.
Use cargo's "--manifest-path" option instead of "cd" in the recipes.

Signed-off-by: David Aguilar <davvid@gmail.com>
Acked-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>