nix: package: replace sdbus-cpp with sdbus-cpp_2 (closes #286)

[spikespaz]

Replace XDPH's package argument sdbus-cpp with sdbus-cpp_2.

This is my less preferred option.

Let's discuss in #286.

Also see #288.

This PR should be merged over that one in the event that sdbus-cpp_2 is a permanent addition to Nixpkgs (and sdbus-cpp stays for dependents on 1.5.x).

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Original:

In the default overlay, override XDPH's package arguments to say that sdbus-cpp = sdbus-cpp_2 which was introduced in a prior commit.

Let's discuss in #286.

This PR is my preference, until we do more digging in Nixpkgs to see if sdbus-cpp_2 is a permanent name, or if they're phasing it out once other package updates are merged.

[spikespaz]

cc @fufexan