fontconfig: Fix missing default fontconfig files (#7045)

This adds the fontconfig's default files to
`home-path/etc/fonts/fonts.conf` and `home-path/etc/fonts/conf.d/*`

modules/misc/fontconfig.nix

@@ -96,6 +96,19 @@ in
       # trying to write to a read-only location.
       (pkgs.runCommandLocal "dummy-fc-dir1" { } "mkdir -p $out/lib/fontconfig")
       (pkgs.runCommandLocal "dummy-fc-dir2" { } "mkdir -p $out/lib/fontconfig")
+      # Provide fontconfig default files from /etc/fonts/
+      (pkgs.runCommand "fontconfig-conf" { } ''
+        dst=$out/etc/fonts/conf.d
+        mkdir -p $dst
+
+        # fonts.conf
+        ln -s ${pkgs.fontconfig.out}/etc/fonts/fonts.conf \
+              $dst/../fonts.conf
+
+        # fontconfig default config files
+        ln -s ${pkgs.fontconfig.out}/etc/fonts/conf.d/*.conf \
+              $dst/
+      '')
     ];
 
     home.extraProfileCommands = ''

tests/modules/misc/fontconfig/no-font-package.nix

@@ -7,5 +7,7 @@
 
   nmt.script = ''
     assertPathNotExists home-path/lib/fontconfig/cache
+    assertLinkExists home-path/etc/fonts/fonts.conf
+    assertDirectoryExists home-path/etc/fonts/conf.d
   '';
 }