Record passed args in Command even on platforms not supporting process spawning

[bjorn3]

This would be useful when running a program that normally executes a different program at the end for example wasm32-wasi. If the program debug prints the std::process::Command on errors that would make it easy for the end user to run the command outside of the wasi sandbox themselves. In my particular case I compiled rustc for wasi and want to copy-paste the linker invocations once all object files are written.

[bjorn3]

Implemented in #123633