Fix test configs: use corepack instead of pnpm nix package

Enable DEVBOX_COREPACK_ENABLED in test devbox.json so pnpm is
available via corepack (bundled with nodejs) rather than as a
separate nix package.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mikeland73

testscripts/add/add_jspm.test.txt

@@ -1,9 +1,7 @@
 # Test adding and removing JS package manager packages
 
-exec devbox init
-
-# Add nodejs and pnpm first so the JS package manager is available
-exec devbox add nodejs_22 pnpm
+# Add nodejs first so the JS package manager is available
+exec devbox add nodejs_22
 
 # Add a pnpm-managed JS package
 exec devbox add pnpm:vercel
@@ -20,22 +18,23 @@ json.superset devbox.json expected_devbox_after_rm.json
 
 -- devbox.json --
 {
-  "packages": []
+  "packages": [],
+  "env": {
+    "DEVBOX_COREPACK_ENABLED": "true"
+  }
 }
 
 -- expected_devbox_after_add.json --
 {
   "packages": [
     "nodejs_22@latest",
-    "pnpm@latest",
     "pnpm:vercel@latest"
   ]
 }
 
 -- expected_devbox_after_rm.json --
 {
   "packages": [
-    "nodejs_22@latest",
-    "pnpm@latest"
+    "nodejs_22@latest"
   ]
 }

testscripts/jspm/devbox.json

@@ -1,9 +1,11 @@
 {
   "packages": [
     "nodejs_22@latest",
-    "pnpm@latest",
     "pnpm:vercel@latest"
   ],
+  "env": {
+    "DEVBOX_COREPACK_ENABLED": "true"
+  },
   "shell": {
     "scripts": {
       "run_test": "vercel --version && echo 'JSPM test passed'"