[nextest-runner] add support for custom targets

[sunshowers]

Pull in target-spec version 2, which implements support for custom targets.

Note that custom targets are not supported in platform overrides, just in --target/CARGO_BUILD_TARGET/build.target.

[codecov]

Codecov Report

Merging #891 (704cd7f) into main (1b3a820) will increase coverage by 0.25%.
The diff coverage is 89.02%.

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
+ Coverage   78.37%   78.62%   +0.25%     
==========================================
  Files          67       67              
  Lines       13673    13949     +276     
==========================================
+ Hits        10716    10968     +252     
- Misses       2957     2981      +24     

Impacted Files	Coverage Δ
cargo-nextest/src/dispatch.rs	84.05% <0.00%> (ø)
nextest-runner/src/errors.rs	30.73% <ø> (ø)
nextest-runner/src/cargo_config/target_triple.rs	87.93% <86.47%> (+1.26%)	⬆️
nextest-runner/src/cargo_config/discovery.rs	91.63% <92.30%> (+0.06%)	⬆️
nextest-runner/src/cargo_config/env.rs	96.85% <100.00%> (+0.16%)	⬆️
nextest-runner/src/cargo_config/test_helpers.rs	100.00% <100.00%> (ø)
nextest-runner/src/config/test_helpers.rs	97.56% <100.00%> (+0.06%)	⬆️
nextest-runner/src/list/binary_list.rs	97.42% <100.00%> (+<0.01%)	⬆️
nextest-runner/src/list/test_list.rs	92.10% <100.00%> (+0.01%)	⬆️
nextest-runner/src/target_runner.rs	94.64% <100.00%> (-0.07%)	⬇️

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