C#: support linux-arm64 in autobuild script

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

csharp/tools/autobuild.sh

@@ -2,7 +2,7 @@
 
 set -eu
 
-if [ "$CODEQL_PLATFORM" != "linux64" ] && [ "$CODEQL_PLATFORM" != "osx64" ] ; then
+if [ "$CODEQL_PLATFORM" != "linux64" ] && [ "$CODEQL_PLATFORM" != "linux-arm64" ] && [ "$CODEQL_PLATFORM" != "osx64" ] ; then
     echo "Automatic build detection for $CODEQL_PLATFORM is not implemented."
     exit 1
 fi