CI: Swap the download location to ftp.gnu.org

Looks like ftpmirror.gnu.org is not reliable (it redirects requests to
other mirror sites, and some of those sites are broken); let's try
ftp.gnu.org instead.

.github/workflows/ci.yml

@@ -108,7 +108,7 @@ jobs:
             guix-cache-
 
       - name: Install Guix
-        uses: sigprof/guix-install-action@463ba86bab4964bf6204befb9edb1145d51e1459
+        uses: sigprof/guix-install-action@0fe71536357cf2a15be1960cd2dbce2cf89ba06a
 
       - name: Ensure no locale warning
         working-directory: .