let format.sh try system clang-format in case it is the correct version

contrib/format.sh

@@ -1,5 +1,21 @@
 #!/usr/bin/env bash
 
+CLANG_FORMAT_DESIRED_VERSION=11
+
+binary=$(which clang-format-$CLANG_FORMAT_DESIRED_VERSION 2>/dev/null)
+if [ $? -ne 0 ]; then
+    binary=$(which clang-format 2>/dev/null)
+    if [ $? -ne 0 ]; then
+        echo "Please install clang-format version $CLANG_FORMAT_DESIRED_VERSION and re-run this script."
+        exit 1
+    fi
+    version=$(clang-format --version)
+    if [[ ! $version == *"clang-format version $CLANG_FORMAT_DESIRED_VERSION"* ]]; then
+        echo "Please install clang-format version $CLANG_FORMAT_DESIRED_VERSION and re-run this script."
+        exit 1
+    fi
+fi
+
 # TODO: readlink -e is a GNU-ism
 cd "$(readlink -e $(dirname $0)/../)"
-clang-format-11 -i $(find jni daemon llarp include pybind | grep -E '\.[hc](pp)?$') &> /dev/null
+$binary -i $(find jni daemon llarp include pybind | grep -E '\.[hc](pp)?$') &> /dev/null