Only skip yarn setup for PNP

This was breaking the vue-cli templates and seems to be purely for pnp purposes:

https://github.com/storybookjs/storybook/pull/21046/files#diff-76aad599d2311ab4fec53db693f2e85d4cb7f798cc37357dfe7839997caef2ba

scripts/sandbox-generators/generate-sandboxes.ts

@@ -142,7 +142,7 @@ const runGenerators = async (
 
         // We do the creation inside a temp dir to avoid yarn container problems
         const createBaseDir = directory();
-        if (!script.includes('yarn')) {
+        if (!script.includes('pnp')) {
           await setupYarn({ cwd: createBaseDir });
         }