ci: revert pnpm/action-setup to v2

pnpm/action-setup@v4 errors with "Multiple versions of pnpm specified"
because the workflow passes version: 9 while AlistGo/desktop package.json
pins packageManager pnpm@8.6.10. The lockfile is 9.0 so version: 9 is the
correct one; v2 respects the input and ignores the field. checkout and
setup-node stay on v4.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: okatu-loli

.github/workflows/release.yml

@@ -55,7 +55,7 @@ jobs:
         with:
           workspaces: "./src-tauri -> target"
 
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v2
         with:
           version: 9
           run_install: false
@@ -131,7 +131,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v2
         with:
           version: 9
           run_install: false

.github/workflows/release_beta.yml

@@ -74,7 +74,7 @@ jobs:
         with:
           workspaces: "./src-tauri -> target"
 
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v2
         with:
           version: 9
           run_install: false
@@ -172,7 +172,7 @@ jobs:
         with:
           ref: main
 
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v2
         with:
           version: 9
           run_install: false