ci: Update setup-pdm action version

Author: pawamoy

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
       uses: actions/checkout@v2
 
     - name: Set up PDM
-      uses: pdm-project/setup-pdm@v2.5
+      uses: pdm-project/setup-pdm@v2.6
       with:
         python-version: "3.8"
 
@@ -84,7 +84,7 @@ jobs:
       uses: actions/checkout@v2
 
     - name: Set up PDM
-      uses: pdm-project/setup-pdm@v2.5
+      uses: pdm-project/setup-pdm@v2.6
       with:
         python-version: ${{ matrix.python-version }}