`RUSTFLAGS` via cargo config

I tried to integrate `cargo-dist` into my release workflow and hit a roadblock in form of my required rustflags.

In my cargo config I set a rustflag to compile the `uuid` crate with unstable features to be able to use UUIDv7.

`cargo-dist`'s usage of the environment variable completely overwrites the cargo configuration though.

Right now I'm resorting to allowing a dirty CI state and adding the `RUSTFLAGS` env-var with the appropriate value, since `cargo-dist` preserves those.

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I personally see two possible solutions to this:

1. `cargo-dist` parses the cargo config and appends any rustflags to the environment variable it sets
2. `cargo-dist` adds the ability to add arbitrary environment variables

Both of these would be nice features tbh

Start by tracing where cargo-dist sets RUSTFLAGS during its release workflow. Compare preserving cargo-config rustflags with supporting arbitrary environment variables, then define completion as allowing the required rustflags to survive without a dirty CI workaround.

Written by the indexing model from the issue text.