BLD: Add conda cache to Windows azure CI

Author: trallard

ci/azure/windows.yml

@@ -43,10 +43,19 @@ jobs:
       displayName: 'Add conda to PATH'
     - script: conda update -q -n base conda
       displayName: 'Update conda'
-
+    - task: Cache@2
+      displayName: Use cached Anaconda environment
+      inputs:
+        key: 'conda | "$(Agent.OS)" | "$(ENV_FILE)"'
+        restoreKeys: |
+          python | "$(Agent.OS)"
+          python
+        path: $(CONDA_CACHE_DIR)
+        cacheHitVar: CONDA_CACHE_RESTORED
     - bash: |
         conda env create -q --file ci\\deps\\azure-windows-$(CONDA_PY).yaml
       displayName: 'Create anaconda environment'
+      condition: eq(variables.CONDA_CACHE_RESTORED, 'false')
     - bash: |
         source activate pandas-dev
         conda list