Feature request: Allow custom targets that do not adhere to target-lexicon

### Discussed in https://github.com/nextest-rs/nextest/discussions/3224 <div type='discussions-op-text'> ^{Originally posted by **mhatzl** March 27, 2026} ### Motivation At Ferrous Systems we have some custom Rust targets that we use for testing purposes, which do not pass the target naming scheme detected by the `target-lexicon` crate used indirectly in `cargo-nextest` via `target-spec`. Therefore it is not possible to use `cargo-nextest` for those targets, because I get e.g. `error: unknown triple string: aarch64-a53-ferrocene.facade` I could not find anything in the docs that would allow to specify custom targets. ### Proposal Ideally, I'd like to define custom targets in the nextest configuration file, because it then does not require to add support to `target-spec` or `target-lexicon` for every new target that does not strictly follow the Rust target naming scheme. If this is something that would get merged, I'd try to get a PR up myself. ### Alternatives I locally went down the chain and *patched* `target-lexicon` to explicitly detect one of our test targets. This kind of worked, but `target-spec` currently uses `target-lexicon` directly with v0.13.5 and indirectly as re-export from `cfg-expr` at v0.13.3. This makes it quite hard to add support for new targets via `target-lexicon`, because it would require a bump in `target-lexicon`, `cfg-expr`, and `target-spec`. ### Additional context _No response_</div>