Fix sysinfo CPU brand output

[Friz64]

Objective

sysinfo was updated to 0.30 in #11071. Ever since then the cpu field of the SystemInfo struct that gets printed every time one starts an bevy app has been empty. This is because the following part of the sysinfo migration guide was overlooked:

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Cpu changes

Information like Cpu::brand, Cpu::vendor_id or Cpu::frequency are not set on the "global" CPU.

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Solution

• Get the CPU brand information from a specific CPU instead. In this case, just choose the first one. It's theoretically possible for different CPUs to have different names, but in practice this doesn't really happen I think. Even Intel's newer hybrid processors use a uniform name for all CPUs in my experience.
• We can use this opportunity to also update our sysinfo::System initialization here to only fetch the information we're interested in.

[rparrett]

Looks good / works for me.

[alice-i-cecile]

Nice catch!