gnu: feh, newsboat: Inline top-level reference to 'curl'.

These references introduced top-level circular dependencies.
Furthermore, they were inaccurate since only CURL_CA_BUNDLE is honored.

* gnu/packages/image-viewers.scm (feh)[native-search-paths]: Remove
top-level reference to 'curl'; inline "CURL_CA_BUNDLE" spec.
* gnu/packages/syndication.scm (newsboat)[native-search-paths]:
Likewise.

gnu/packages/image-viewers.scm

@@ -287,7 +287,11 @@ YouTube videos without requiring API and opens/downloads them using mpv/ytdl.")
     (native-search-paths
      ;; Feh allows overriding the libcurl builtin CA path (unset in Guix)
      ;; with the same variable as the `curl` command line HTTP tool.
-     (package-native-search-paths curl))
+     (list (search-path-specification
+            (variable "CURL_CA_BUNDLE")
+            (file-type 'regular)
+            (separator #f)                             ;single entry
+            (files '("etc/ssl/certs/ca-certificates.crt")))))
     (synopsis "Fast and light imlib2-based image viewer")
     (description
       "feh is an X11 image viewer aimed mostly at console users.

gnu/packages/syndication.scm

@@ -291,7 +291,11 @@ cards.")
            (assoc-ref gnu:%standard-phases 'install)))))
     (native-search-paths
      ;; Newsboat respects CURL_CA_BUNDLE.
-     (package-native-search-paths curl))
+     (list (search-path-specification
+            (variable "CURL_CA_BUNDLE")
+            (file-type 'regular)
+            (separator #f)                        ;single entry
+            (files '("etc/ssl/certs/ca-certificates.crt")))))
     (home-page "https://newsboat.org/")
     (synopsis "Text-mode RSS and Atom feed reader with podcast support")
     (description "Newsboat is a feed reader for @dfn{RSS} and @dfn{Atom}, XML