sidebar_gen: force C locale

If we don't, find output can change depending on system locale, which
will produce inconsistent results.

scripts/gen_docs_sidebar.sh

@@ -8,6 +8,7 @@
 # The script will ignore any files that are not markdown files.
 # Usage: bash scripts/gen_docs_sidebar.sh > docs/_sidebar.md
 
+export LANG=C
 {
 echo "* [Readme](/)"