Filter build-chroot-dirs entries that conflict with derivation outputs

Fixes https://github.com/NixOS/nixpkgs/issues/9504.

Note that this means we may have a non-functional /bin/sh in the
chroot while rebuilding Bash or one of its dependencies. Ideally those
packages don't rely on /bin/sh though.

src/libstore/build.cc

@@ -1998,11 +1998,11 @@ void DerivationGoal::startBuilder()
             }
         }
 
-        /* If we're repairing or checking, it's possible that we're
+        /* If we're repairing, checking or rebuilding part of a
+           multiple-outputs derivation, it's possible that we're
            rebuilding a path that is in settings.dirsInChroot
            (typically the dependencies of /bin/sh).  Throw them
            out. */
-        if (buildMode != bmNormal)
         for (auto & i : drv->outputs)
             dirsInChroot.erase(i.second.path);