[clang-tidy] fix None tmpdir when exporting fixes in run-clang-tidy

Differential https://reviews.llvm.org/D145477 removed the check for `(yaml and args.export_fixes)` in line 303 to skip looking for the `clang-apply-replacements` binary. However, the `tmpdir` variable was set in this true branch when exporting fixes and therefore is `None` when invoking run-clang-tidy with `run-clang-tidy -p . -export-fixes fixes.yaml`.

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D157773