Use cmd.exe to export env vars before commands

Use cmd.exe to export env vars before commands

Microsoft NMake runs the entire command block in a single `cmd.exe`
session, so environment variables exported with `set` are seen by the
following commands. C++Builder's `make.exe` doesn't use `cmd.exe` but
executes each command line separately. While it emulates only certain
shell features itself, `set` isn't one of them.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13540)