std: explicitly handle error.UnexpectedExitCode in build_runner

RunStep on unexpected exit code used to return error.UncleanExit, which
was confusing and unclear. When it was changed, the error handling code
in build_runner was not modified, which produced an error trace.

This commit explicitly handles error.UnexpectedExitCode in build_runner
so that the behavior now matches that of zig 0.8.1 after which it was
regressed.

lib/std/special/build_runner.zig

@@ -205,7 +205,7 @@ pub fn main() !void {
             error.InvalidStepName => {
                 return usageAndErr(builder, true, stderr_stream);
             },
-            error.UncleanExit => process.exit(1),
+            error.UnexpectedExitCode, error.UncleanExit => process.exit(1),
             else => return err,
         }
     };