[sled-diagnostics] log collection should happen on u.2s

[papertigers]

Currently we collect logs into a zip file that lives on m.2s we should instead use u.2s as temporary storage.

omicron/sled-storage/src/resources.rs

Lines 185 to 192 in a016336

	/// Return the directories that can be used for temporary sled-diagnostics
	/// file storage.
	pub fn all_sled_diagnostics_directories(&self) -> Vec<Utf8PathBuf> {
	// These directories are currently used for tempfile storage when
	// zipping up zone logs before shuffling them off to a nexus collecting
	// a support bundle.
	self.all_m2_mountpoints(M2_DEBUG_DATASET).into_iter().collect()
	}

The logic in sled-diagnostics currently selects the first item in the vec, we could select one at random or we could look for which one has the most free space.