home-manager: resolve default configuration file path

Home Manager needs an absolute and resolved path to its configuration
file. The default configuration path is absolute but not necessarily
resolved. For example, some users may have `~/.config` be a symbolic
link to somewhere else. We therefore run the default configuration
path through the `realpath` tool to resolve it.

Fixes #304

home-manager/home-manager

@@ -32,7 +32,7 @@ function setConfigFile() {
     for confFile in "$defaultConfFile" \
                     "$HOME/.nixpkgs/home.nix" ; do
         if [[ -e "$confFile" ]] ; then
-            HOME_MANAGER_CONFIG="$confFile"
+            HOME_MANAGER_CONFIG="$(realpath "$confFile")"
             return
         fi
     done