fix(libflux): only compile flux-core with strict if enabled in flux

[davidkna]

Checklist

By inheriting the value of strict it is simpler to build flux in cases where warnings are acceptable without needing to patch the flux-core Cargo.toml or source.

The changes can be tested by adding a dead function to flux-core e.g. fn dead_code() {} and compiling flux with --no-default-features.

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