Windows: improve external checks and env-var help

Add test for `cat` external

Fix the env-var to read the correct EASYRSA_OPENSSL name

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Windows: make builds easier by using a matching dir structure

Now the entire distro/windows/ dir contents can be copied into a staging
dir during release prep.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>