update pip before installing azdev (Azure#3338)

Author: fengzhou-msft

azure-pipelines.yml

@@ -54,6 +54,7 @@ jobs:
       # clone azure-cli
       git clone -q --single-branch -b dev https://github.com/Azure/azure-cli.git ../azure-cli
 
+      python -m pip install -U pip
       pip install -q azdev==0.1.31
 
       azdev setup -c ../azure-cli -r ./
@@ -138,6 +139,7 @@ jobs:
         # clone azure-cli
         git clone --single-branch -b dev https://github.com/Azure/azure-cli.git ../azure-cli
 
+        python -m pip install -U pip
         pip install azdev==0.1.31
 
         azdev --version