Some rust tasks broken under fish

[WeetHet]

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• [X] Completed

Describe the bug / provide steps to reproduce it

Open any rust project with fish as your shell, spawn some task that uses ZED_CUSTOM_RUST_PACKAGE. Get

fish: Variables cannot be bracketed. In fish, please use {$ZED_CUSTOM_RUST…}.
cargo test -p ${ZED_CUSTOM_RUST_PACKAGE}
               ^

⏵ Task `cargo test -p rust-playground` finished with non-zero error code: 127

Environment

Zed: v0.133.2 (Zed Preview) OS: macOS 14.4.1 Memory: 8 GiB Architecture: aarch64

If applicable, add mockups / screenshots to help explain present your vision of the feature

If applicable, attach your ~/Library/Logs/Zed/Zed.log file to this issue.

No response

[osiewicz]

Related to: #10964

[romans-ovo]

$ZED_CUSTOM_RUST_PACKAGE should work for any shell.

Unless some exotic syntax is used (e.g. ${NAME}_id) bracketing variables do not offer any advantage.