feat: spirv-builder takes optional path to target spec JSON file

[schell]

This adds an optional field to SpirvBuilder that will set the "target specification" file. This will enable cargo-gpu to point to these files, which can be stored in its cache directory.

Relations

• error: target path ".../spirv-unknown-vulkan1.2.json" is not a valid file cargo-gpu#10
• $CARGO_MANIFEST_DIR "leakage" (used at runtime) due to target-specs vendoring. #22
• Consider build CLI (e.g. cargo gpu) mapping spirv-std deps to toolchain & backend versions. EmbarkStudios/rust-gpu#1137

[LegNeato]

rustfmt

[schell]

Ah, yes - I forgot that since we can only build on macOS with --release, my editor's LSP doesn't work on this project...