servo: Merge #18953 - Fix './mach clean-nightlies --keep 3' not keeping anything (from servo:keep); r=jdm

This affects CI, re-downloading Nightly for every build.

Source-Repo: https://github.com/servo/servo
Source-Revision: d4bdb8b2936d3b47841103fd5c843860ef5c7474

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 35889bece1fe60de22df28261f1c45f1ad8219cb

servo/python/servo/bootstrap_commands.py

@@ -308,7 +308,10 @@ class MachCommands(CommandBase):
                 stdout, _ = cmd.communicate()
                 for line in stdout.splitlines():
                     if line.startswith(b"+") and not line.startswith(b"+++"):
-                        to_keep.add(line[1:])
+                        line = line[len(b"+"):]
+                        if line.startswith(b"nightly-"):
+                            line = line[len(b"nightly-"):]
+                        to_keep.add(line)
 
         removing_anything = False
         for tool in ["rust", "cargo"]: