This enables DEBUG_RUSTFLAGS with the appropriate debuginfo settings.

The empty RUSTFLAGS variable is required workaround to avoid double
compilation during package function call. The issue is that the
behavior of the current implementation of buildenv_debugflags is not
idempotent, so consecutive calls will append the same flags again
leading to cargo consider the build inputs to have changed.

Signed-off-by: Levente Polyak <anthraxx@archlinux.org>