unconditional dependency on alloc is a problem in no_std/no_alloc

[Crzyrndm]

alloc is only required for tests(?) so just make no_std conditional on tests|"std" instead

The CI no_std check failed to catch this because you're not building an executable (dependencies are not being checked). Minimal executable has been added and CI now builds that

depends on https://github.com/lights0123/printf-compat/pull/5 to fix the clippy lints.