Clear cached store connections when fetching lint warnings

As I'm seeing the inferior process crash with [1] just after fetching the
derivation lint warnings.

This change appears to help, although it's probably just a workaround. When
there's more packages/derivations, the caches might need clearing while
fetching the derivation lint warnings, or this will need to be split across
multiple processes.

1: Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS

guix-data-service/jobs/load-new-guix-revision.scm

@@ -561,8 +561,23 @@ WHERE job_id = $1")
               '()
               (with-time-logging (simple-format #f "getting ~A lint warnings"
                                                 name)
-                (inferior-eval-with-store inf store (lint-warnings-for-checker
-                                                     name)))))))
+                (format (current-error-port)
+                        "inferior heap size: ~a MiB~%"
+                        (round
+                         (/ (inferior-eval '(assoc-ref (gc-stats) 'heap-size) inf)
+                            (expt 2. 20))))
+
+                (let ((warnings
+                       (inferior-eval-with-store
+                        inf
+                        store
+                        (lint-warnings-for-checker name))))
+
+                  ;; Clean the cached store connections, as there are caches
+                  ;; associated with these that take up lots of memory
+                  (inferior-eval '(hash-clear! %store-table) inf)
+
+                  warnings))))))
       checkers))))
 
 (define (all-inferior-package-derivations store inf packages)