Add env var to override clippy-driver location

[NyCodeGHG]

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Hi!
I'm currently working on a problem with our rust support in NixOS/nixpkgs.
We need a way to point cargo-clippy to a wrapper of clippy-driver which provides it with a correct sysroot.
I've came up with this patch to solve it, but we would want to upstream it, so here I am :)
Feel free to let me know what you think of this or if you have a better idea.

Relevant: NixOS/nixpkgs#434028

[rustbot]

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[samueltardieu]

Closing since this after discussion on the NixOS side this can be resolved with nixpkgs wrapper.

@NyCodeGHG Don't hesitate to reopen if that's not the case.

[Jarcho]

From reading the issue I'm not really clear why you don't just have a wrapper around both cargo-clippy and clippy-driver that sets SYSROOT.

Edit: I've been beaten to this.