Make PeachPy processes deterministic:

  https://github.com/Maratyszcza/PeachPy/issues/88
  https://issues.guix.gnu.org/50672

diff --git a/peachpy/name.py b/peachpy/name.py
index b6a03dc..412079d 100644
--- a/peachpy/name.py
+++ b/peachpy/name.py
@@ -86,13 +86,13 @@ def add_scoped_name(self, scoped_name):
                 self.names[scope_name.name] = scope
         else:
             assert scope_name.name is None
-            self.prenames.setdefault(scope_name.prename, set())
+            self.prenames.setdefault(scope_name.prename, {})
             if subscoped_name:
                 for subscope in iter(self.prenames[scope_name.prename]):
                     if isinstance(subscope, Namespace) and subscope.scope_name is scope_name:
                         subscope.add_scoped_name(subscoped_name)
                         return
-            self.prenames[scope_name.prename].add(scope)
+            self.prenames[scope_name.prename][scope] = None
 
     def assign_names(self):
         # Step 1: assign names to symbols with prenames with no conflicts
'/guix/'>summary</a><a href='/guix/refs/?id=2b25208c81be88913c32e085cfdff1b7777ce833'>refs</a><a class='active' href='/guix/log/gnu/installer/substitutes.scm'>log</a><a href='/guix/tree/gnu/installer/substitutes.scm?id=2b25208c81be88913c32e085cfdff1b7777ce833'>tree</a><a href='/guix/commit/gnu/installer/substitutes.scm?id=2b25208c81be88913c32e085cfdff1b7777ce833'>commit</a><a href='/guix/diff/gnu/installer/substitutes.scm?id=2b25208c81be88913c32e085cfdff1b7777ce833'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/installer/substitutes.scm'>
<input type='hidden' name='id' value='2b25208c81be88913c32e085cfdff1b7777ce833'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=2b25208c81be88913c32e085cfdff1b7777ce833'>root</a>/<a href='/guix/log/gnu?id=2b25208c81be88913c32e085cfdff1b7777ce833'>gnu</a>/<a href='/guix/log/gnu/installer?id=2b25208c81be88913c32e085cfdff1b7777ce833'>installer</a>/<a href='/guix/log/gnu/installer/substitutes.scm?id=2b25208c81be88913c32e085cfdff1b7777ce833'>substitutes.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/installer/substitutes.scm?id=2b25208c81be88913c32e085cfdff1b7777ce833&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-11 21:37:07 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/installer/substitutes.scm?id=8361817bf693742757b096468198626f297bb09e'>install: Discover local substitute servers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/substitutes.scm: New file.
* gnu/installer/newt/substitutes.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* po/guix/POTFILES.in: Add gnu/installer/newt/substitutes.scm.
* gnu/installer/proxy.scm (with-silent-shepherd): Move to ...
* gnu/installer/utils.scm: ... here.
* gnu/installer/record.scm (&lt;installer&gt;)[substitutes-page]: New field.
* gnu/installer/newt.scm (substitutes-page): New procedure,
(newt-installer): register it.
* gnu/installer.scm (installer-steps): Add "substitutes-page" step.
* gnu/system/install.scm (%installation-services): Add avahi-service-type and
enable substitute server discover in guix-service-type.
[&lt;name-service-switch&gt;]: Set it to %mdns-host-lookup-nss.
</span></span></td><td>Mathieu Othacehe</td></tr>