doc: getting_started: Add note for Windows env vars
doc: getting_started: Add note for Windows env vars

Some users might want to inherit their already existing Windows
environment variables into the MSYS2 system. This note explains how to
achieve this.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

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