[hotfix][tests] e2e/common.sh: add -type parameter to find

This prevents "Is a directory" error when running "head":
Checking for exceptions...
Found exception in log files; printing first 500 lines; see full logs for details:
head: error reading '/home/vsts/work/1/s/flink-dist/target/flink-1.12-SNAPSHOT-bin/flink-1.12-SNAPSHOT/log/': Is a directory

flink-end-to-end-tests/test-scripts/common.sh

@@ -367,7 +367,7 @@ function check_logs_for_errors {
       | grep -ic "error" || true)
   if [[ ${error_count} -gt 0 ]]; then
     echo "Found error in log files; printing first 500 lines; see full logs for details:"
-    find $FLINK_DIR/log/ -exec head -n 500 {} \;
+    find $FLINK_DIR/log/ -type f -exec head -n 500 {} \;
     EXIT_CODE=1
   else
     echo "No errors in log files."
@@ -404,7 +404,7 @@ function check_logs_for_exceptions {
    | grep -ic "exception" || true)
   if [[ ${exception_count} -gt 0 ]]; then
     echo "Found exception in log files; printing first 500 lines; see full logs for details:"
-    find $FLINK_DIR/log/ -exec head -n 500 {} \;
+    find $FLINK_DIR/log/ -type f -exec head -n 500 {} \;
     EXIT_CODE=1
   else
     echo "No exceptions in log files."
@@ -426,7 +426,7 @@ function check_logs_for_non_empty_out_files {
    | grep "." \
    > /dev/null; then
     echo "Found non-empty .out files; printing first 500 lines; see full logs for details:"
-    find $FLINK_DIR/log/ -name '*.out' -exec head -n 500 {} \;
+    find $FLINK_DIR/log/ -type f -name '*.out' -exec head -n 500 {} \;
     EXIT_CODE=1
   else
     echo "No non-empty .out files."