fix(ci/toolchain): enforce rust 1.90 in preview workflow

[zemaj]

Summary

• switch Preview Build to rustup toolchain 1.90.0, explicitly add the matrix target, and bump the cache key to rust-1.90 so reruns pick up the new toolchain
• align issue-code and upstream-merge workflows with Rust 1.90.0 to keep auxiliary jobs on the same compiler
• reference ci: update preview build to Rust 1.90.0 #364 plus dependent PRs (fix(sandbox): keep full-auto workspace-writable #357, fix(tui): preserve full-width IME spaces #358, fix(responses): send stable beta header #362, fix(auto-drive): retry on empty stream output #363, fix(rollout): favor newest session mtime #365, fix(tui/mcp): keep menu selection in view #366, fix(homebrew): refuse placeholder versions #367, fix(tui/ime): drop repeat events during composition #368) that have been failing with E0463 due to the old toolchain

Testing

• ./build-fast.sh

Refs: #364, #357, #358, #362, #363, #365, #366, #367, #368

[github-actions]

Preview Build

You can run this right now! In your terminal just use:

code preview toolchain-preview-build

This will run the version of Code we've created for this issue.

Direct downloads:

• Linux x86_64 (.zst): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-x86_64-unknown-linux-musl.zst
• Linux x86_64 (.tar.gz): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-x86_64-unknown-linux-musl.tar.gz
• Linux aarch64 (.zst): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-aarch64-unknown-linux-musl.zst
• Linux aarch64 (.tar.gz): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-aarch64-unknown-linux-musl.tar.gz
• macOS x86_64 (.zst): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-x86_64-apple-darwin.zst
• macOS x86_64 (.tar.gz): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-x86_64-apple-darwin.tar.gz
• macOS arm64 (.zst): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-aarch64-apple-darwin.zst
• macOS arm64 (.tar.gz): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-aarch64-apple-darwin.tar.gz
• Windows x86_64 (.zip): https://github.com/just-every/code/releases/download/preview-toolchain-preview-build/code-x86_64-pc-windows-msvc.exe.zip

After you run it, please comment here and let me know if it does what you need.

[zemaj]

Preview Build is green across all five targets with Rust 1.90.0 + MUSL. Ready to merge so we can rerun the dependent preview builds (#357/#358/#362/#363/#365/#366/#367/#368).