appveyor.yml: clarify conditions for building the plain configuration

appveyor.yml: clarify conditions for building the plain configuration

The "plain" configuration is only meant to be built for an '[extended tests]'
commit, or on the master branch.  This isn't at all clear from the
scripts, and furthermore, we "skip" the plain configuration by running
the OpenSSL configuration script...  and then nothing more.

Instead, we use AppVeyor configuration issues to specify when and when
not to build the "plain" configuration, and leave it to the scripts to
do the right thing using only $env:EXTENDED_TESTS.

Fixes #7958

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13537)