cargo-build.sh: support optimization=plain
meson 0.64.0 added optimization=plain. It means that no optimization flags are explicitly passed to the compiler. This is useful in situations where the environment wants to control compiler flags (e.g. nix, rpm, etc), typically via CFLAGS, CXXFLAGS, etc.
Make sure to handle optimization=plain so that cargo-build.sh doesn't exit early.
Signed-off-by: Stefan Hajnoczi stefanha@redhat.com