feat: Set default CI environment variables (#40)

This allows code being run by the CI server to detect that it's running
inside a CI server.

The `CI` environment variable is exposed by most major CI servers, but
having a vendor specific environment variable (i.e. `STRIDER`) is also
common practice.

For details see issue Strider-CD/strider#892

lib/index.js

@@ -372,7 +372,10 @@ Runner.prototype = {
           return next(err);
         }
 
-        var env = {};
+        var env = {
+          CI: 'true',
+          STRIDER: 'true',
+        };
 
         if (config.envKeys) {
           env.STRIDER_SSH_PUB = config.pubkey;