Bootstrap using `set_times` on a read-only file

[ChrisDenton]

@jieyouxu noticed that bootstrap is now failing on a clean Windows build:

thread 'main' panicked at src\core\download.rs:711:13:
llvm_config_file.set_times(file_times) failed with Access is denied. (os error 5)

This fails due to the file being opened as read-only:

rust/src/bootstrap/src/core/download.rs

Lines 709 to 711 in cb12b52

	let llvm_config_file = t!(File::open(llvm_config));
	t!(llvm_config_file.set_times(file_times));

This could be fixed using windows specific permissions but it'd be simpler to just use OpenOptions with write(true).