[CI] run-pylint: Use pylint if pylint3 is missing

Starting from Ubuntu 19.10 pylint3 was replaced with pylint.

.ci/run-pylint

@@ -4,10 +4,16 @@ set -e
 
 cd "$(git rev-parse --show-toplevel)"
 
+# pylint3 was replaced with pylint from Ubuntu 19.10
+PYLINT=$(command -v pylint3)
+if [ -z "$PYLINT" ]; then
+    PYLINT=$(command -v pylint)
+fi
+
 find . -name \*.py \
     -and -not -path ./LibOS/shim/test/apps/ltp/src/\* \
 | sed 's/./\\&/g' \
-| xargs pylint3 "$@" \
+| xargs "${PYLINT}" "$@" \
     Pal/src/host/Linux-SGX/signer/pal-sgx-get-token \
     Pal/src/host/Linux-SGX/signer/pal-sgx-sign \
     .ci/prfilter