Upgrade pip for linux worker build (#782)

Author: Hazhzeng

pack/scripts/nix_deps.sh

@@ -2,6 +2,8 @@
 
 python -m venv .env
 source .env/bin/activate
+python -m pip install --upgrade pip
+
 python -m pip install .
 
 python -m pip install . azure-functions --no-compile --target "$BUILD_SOURCESDIRECTORY/deps"