Move netrcFile to Settings

Also get rid of Settings::processEnvironment(), it appears to be
useless.

src/libstore/globals.hh

@@ -16,8 +16,6 @@ struct Settings {
 
     Settings();
 
-    void processEnvironment();
-
     void loadConfFile();
 
     void set(const string & name, const string & value);
@@ -193,6 +191,10 @@ struct Settings {
        build settings */
     Path preBuildHook;
 
+    /* Path to the netrc file used to obtain usernames/passwords for
+       downloads. */
+    Path netrcFile;
+
 private:
     SettingsMap settings, overrides;