Dependency tweaks for building on Android

[metasim]

A couple Cargo.toml tweaks to handle modern Android targets.

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Pull request overview

Updates workspace and crate-level Cargo dependency configuration to support building CubeCL on Android targets (especially where no_std/alloc combinations matter).

Changes:

• Enable serde_json’s alloc feature in the workspace dependency definition.
• Extend cubecl-runtime’s target-gated “persistent cache” dependency set to include target_os = "android".

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Cargo.toml	Adds serde_json alloc feature at the workspace level for broader no_std/Android compatibility.
crates/cubecl-runtime/Cargo.toml	Includes Android in the target-gated dependency block used for persistent cache support.

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[laggui]

LGTM! Just a minor comment but approving in advance