Update dependency on nix to 0.28

[neumann-paulmann]

The current dependency on nix 0.26 results in multiple crate versions for bitflags.
The latest release of nix (0.28) fixes this.

[sirhcel]

Thank you for pointing this out! I see. Does this affect how serialport works for you? Or do you notice this "just" in the dependency tree?

[neumann-paulmann]

It currently does not have any measurable impact on the performance of the software I use.
So if that's what you're asking, yes it's "just" an aesthetic issue.

[sirhcel]

Thank you for stating this! I would like to postpone this to the next major release then for not breaking dependencies relying on MSRV 1.59.0 (see #176 (comment)).