PathSubstitutionGoal: Clean up pipe

If there were many top-level goals (which are not destroyed until the
very end), commands like

  $ nix copy --to 'ssh://localhost?remote-store=/tmp/nix' \
    /run/current-system --no-check-sigs --substitute-on-destination

could fail with "Too many open files". So now we do some explicit
cleanup from amDone(). It would be cleaner to separate goals from
their temporary internal state, but that would be a bigger refactor.

src/libstore/build/substitution-goal.hh

@@ -14,7 +14,7 @@ struct PathSubstitutionGoal : public Goal
     StorePath storePath;
 
     /* The path the substituter refers to the path as. This will be
-     * different when the stores have different names. */
+       different when the stores have different names. */
     std::optional<StorePath> subPath;
 
     /* The remaining substituters. */
@@ -79,6 +79,8 @@ public:
     /* Callback used by the worker to write to the log. */
     void handleChildOutput(int fd, const string & data) override;
     void handleEOF(int fd) override;
+
+    void cleanup() override;
 };
 
 }