fix shebang test failure introduced at r41873

On Windows, it doesn't use /usr/bin/env.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41891 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
fix PATH environ related test failure introduced at r41873

On Windows PATH_SEPARATOR is ';'.
Therefore it can't be hardcoded.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41892 b2dd03c8-39d4-4d8f-98ff-823fe69b080e