fix(macros): cache rustc -V output

[montyc1999]

The #[tauri::command] macro runs a rustc -V version check for stateful async commands.

Each rustc -V takes 50-100ms on my machine, which can add up to several seconds for a Tauri app with many commands.

To demonstrate this, try adding 100 async commands to the helloworld example like so:
montyc1999@9b0de6b

On my machine, an incremental build with touch examples/helloworld/main.rs && cargo build --example helloworld takes 4.6s without caching, or 1.2s with caching.

[github-actions]

Package Changes Through 237c583

There are 1 changes which include tauri-macros with patch

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
tauri-macros	2.3.0	2.3.1
tauri	2.6.0	2.6.1

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[Legend-Master]

Nice catch, thanks!