Update cfg_aliases dependency to 0.2

[dtolnay]

What does this PR do

This fixes the following notice when running cargo update in any project that depends on the nix crate.

$ cargo update
    Updating crates.io index
note: pass `--verbose` to see 1 unchanged dependencies behind latest

$ cargo update --verbose
    Updating crates.io index
   Unchanged cfg_aliases v0.1.1 (latest: v0.2.0)
note: to see how you depend on a package, run `cargo tree --invert --package <dep>@<ver>`

$ cargo tree --invert --package cfg_aliases
cfg_aliases v0.1.1
[build-dependencies]
└── nix v0.28.0
    └── faketty v1.0.15

Here is the changelog for cfg_aliases 0.2.0: https://github.com/katharostech/cfg_aliases/releases/tag/v0.2.0

The change related to debug_assertions appears to be the impetus for 0.2 to be released as a breaking change, but it does not affect nix as debug_assertions is not used in nix's build.rs.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Thanks!