fix: remove unused PADDEDCELL_SKILLS_DIR env variable

The plugins.load.paths configuration is automatically updated,
so this environment variable is not needed.

install.mjs

@@ -447,7 +447,6 @@ async function configure(env) {
     'export PASS_MGR_PATH="pass_mgr"',
     '',
     '# PaddedCell skills',
-    `export PADDEDCELL_SKILLS_DIR="${join(installDir, 'skills', 'paddedcell')}"`,
   ].join('\n');
 
   const envFile = join(installDir, 'paddedcell.env');