Fix platform detection for FreeBSD (#47839)

eng/build.sh

@@ -36,6 +36,8 @@ runtime_source_feed_key=''
 
 if [ "$(uname)" = "Darwin" ]; then
     target_os_name='osx'
+elif [ "$(uname)" = "FreeBSD" ]; then
+    target_os_name='freebsd'
 else
     target_os_name='linux'
 fi
@@ -380,4 +382,4 @@ if [ "$only_build_repo_tasks" != true ]; then
     MSBuild $_InitializeToolset -p:RepoRoot="$repo_root" ${msbuild_args[@]+"${msbuild_args[@]}"}
 fi
 
-ExitWithExitCode 0
+ExitWithExitCode 0