Since `HOMEBREW_PROCESSOR` is populated using `uname -m`, this will
register as `Intel` even when a native ARM install is running under
Rosetta.

Let's work around this by checking `sysctl -n machdep.cpu.brand_string`.
On my Intel machine:

    ❯ sysctl -n machdep.cpu.brand_string
    Intel(R) Core(TM) i3-1000NG4 CPU @ 1.10GHz

On Apple Silicon:

    brew@HMBRW-A-001-M1-001 ~ % sysctl -n machdep.cpu.brand_string
    Apple M1

The case of a Rosetta installation of Homebrew is already handled below
the proposed change.

Fixes Homebrew/discussions#2434.