fix npx binary resolution: run in workspace, not action directory

npx was running with cwd set to the action's own directory, which has
package.json with "name": "pullfrog". npm treats the local package as
satisfying the request and skips the registry fetch, then fails to find
the binary (sh: 1: pullfrog: not found). use GITHUB_WORKSPACE instead.

Made-with: Cursor

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pullfrog",
-  "version": "0.0.196",
+  "version": "0.0.197",
   "type": "module",
   "bin": {
     "pullfrog": "dist/cli.mjs",