Include linker flags for package note in %build_rustflags
%build_rustflags was used in %cargo_prep, which is executed in %prep,
when %buildsubdir is not set yet. To avoid this, insertion of flags is
moved to an environment variable that is set for %cargo_build and
%cargo_test.
The linker flag gets passed to rustc, and resulting binaries seem to have
note as expected.