Anchor channels.d path to channels.scm's own location, not CWD

"./channels.d" only resolved because guix pull -C channels.scm happened
to be invoked with CWD == this file's directory. Once guix pull loads
this file as a plain module (e.g. from the store, to build its package
cache), CWD is unrelated to the checkout, so scandir returned #f and map
blew up with "Not a list: #f". Use (current-filename) instead so it works
regardless of how/where this file gets loaded from.

channels.scm

@@ -11,9 +11,17 @@
   #:use-module (guix channels)
   #:use-module ((ice-9 ftw) #:select (scandir)))
 
+;; ./channels.d only resolves when the process's CWD happens to be this
+;; file's own directory, which isn't true once guix pull loads this as a
+;; plain module (e.g. from the store) to build its package cache -- anchor
+;; to this file's own location instead.
+(define here (dirname (current-filename)))
+
+(define channels-directory (string-append here "/channels.d"))
+
 (define channel-files
-  (scandir "./channels.d"
+  (scandir channels-directory
     (lambda (file)
       (string-suffix? ".scm" file))))
 
-(append (map (lambda (file) (load (string-append "./channels.d/" file))) channel-files) %default-channels)
+(append (map (lambda (file) (load (string-append channels-directory "/" file))) channel-files) %default-channels)