Rustflags apply to ALL the crates in the graph, while we only need
them for the toplevel crate which is the only one getting linked.
Rustflags are not equal for all crates, this caused cargo to re-build the
same dependency crate multiple times uselessly. After this change, deps
are reused more, making builds faster.
Note that this only applies when sharing the target/ dir for multiple crates
in the repo which is not the default.
2221e1fa93