mach clean-nightlies: don’t remove everything versioned by date

(Dates contain `-`.)

python/servo/bootstrap_commands.py

@@ -317,11 +317,9 @@ class MachCommands(CommandBase):
                 full_path = path.join(base, name)
                 if name.startswith("rust-"):
                     name = name[len("rust-"):]
-                # We append `-alt` if LLVM assertions aren't enabled,
-                # so use just the commit hash itself.
-                # This may occasionally leave an extra nightly behind
-                # but won't remove too many nightlies.
-                if name.partition('-')[0] not in to_keep:
+                if name.endswith("-alt"):
+                    name = name[:-len("-alt")]
+                if name not in to_keep:
                     removing_anything = True
                     if force:
                         print("Removing {}".format(full_path))