Improve cargo-zigbuild zig performance

[breezewish]

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I recently discovered my C++ projects builds notably slower when using cargo-zigbuild, compared with a cmake toolchain I craft manually similar to https://github.com/mrexodia/zig-cross

After some debugging, I found that the initial process of fn execute_compiler (everything before actual zig Command::run) takes heavy amout of time, almost equal to the Command::run itself:

It would be great if the initial time could be reduced. Thanks!

[messense]

Maybe we can cache some of the repeated works.

[breezewish]

I guess most cost comes from spawning the Rust and Zig to checkout their versions. The versions could be checked when zigbuild is invoked and stored in zigcc-xx.sh.