Use NOTIFY_ENVIRONMENT to test if running locally

This matches what we tell people, in the README, to put in their local
environment.sh

It doesn’t matter what Jenkins sets this to, as long as it’s not
`'development'`. We think Jenkins isn’t setting it at all (in which
case the value will be `undefined`)

gulpfile.babel.js

@@ -37,7 +37,7 @@ gulp.task('copy:govuk_template:css', () => gulp.src(paths.template + 'assets/sty
   }))
   .on('error', plugins.sass.logError)
   .pipe(plugins.cssUrlAdjuster({
-    prependRelative: process.env.NOTIFY_ADMIN_ENVIRONMENT == 'config.Development' ? '/static/' : '/',
+    prependRelative: process.env.NOTIFY_ENVIRONMENT == 'development' ? '/static/' : '/',
   }))
   .pipe(gulp.dest(paths.dist + 'stylesheets/'))
 );