RFC: Be able to provide "local configuration" while installing a "local crate"

[xelephas]

Describe the problem you are trying to solve

After #6026, cargo install will only use the configuration file at ~/.cargo/config
However, it would be nice to be able to provide a "local configuration" while installing a "local crate"

Describe the solution you'd like

While installing a local crate using cargo install --path {proj-folder}, either:

Only look for {proj-folder}/.cargo/config,

or

First look for {proj-folder}/.cargo/config, and then fall back to ~/.cargo/config if it doesn't exist.

[dwijnand]

In #6026 @alexcrichton indicated this behaviour could land in a future PR, so I've marked this proposal as accepted.

(Edit: and by "this behaviour" I mean the second one listed, with the fallback.)