NSIS has a default MAX_STR_LEN of 1024. If $ENV{PATH} is longer
the returned string is truncated to an empty string. Its then not
possible to distinguis between the variable not set or too long.
As a result the variable is set with the location where geth and/or
dev tools are installed. This may override any previous set values.

This commit adds support for creating Windows installers to ci.go