ci: bump pnpm/action-setup to v6 for pnpm 11 global bin PATH fix

v4 leaves the v11 isolated global bin dir ($PNPM_HOME/bin) out of PATH,
breaking `pnpm i -g @antfu/ni`. v6 ships pnpm 11 support and the PATH fix.

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v6
         with:
           run_install: false
       - uses: actions/setup-node@v4
@@ -38,7 +38,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v6
         with:
           run_install: false
       - name: Set node ${{ matrix.node }}