Silence pip warning in nix-shell

shell.nix

@@ -14,6 +14,7 @@ stdenv.mkDerivation {
     SOURCE_DATE_EPOCH=$(date +%s)  # so that we can use python wheels
     virtualenv --no-setuptools venv > /dev/null
     export PATH=$PWD/venv/bin:$PATH > /dev/null
+    export PIP_DISABLE_PIP_VERSION_CHECK=1
     pip install -r requirements.txt > /dev/null
   '';
 }