Relax restriction on [admins] in dev local.ini

Currently, `dev/run` relies on the `[admins]` section be last in
`local.ini`, or it will fail to start and log this message:
```
[info] 2023-12-02T21:50:40.510174Z node1@127.0.0.1 <0.262.0> --------
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  No Admin Account Found, aborting startup.
  Please configure an admin account in your local.ini file.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
```

This relaxes that restriction by inserting the admin user just after
the `[admin]` section line, rather than at the end of the file.

dev/run

@@ -702,8 +702,9 @@ def hack_local_ini(ctx, contents):
     else:
         user, pswd = ctx["admin"]
 
-    # this relies on [admin] being the last section at the end of the file
-    contents = contents + "\n%s = %s" % (user, hashify(pswd))
+    contents = contents.replace(
+        "\n[admins]\n", "\n[admins]\n%s = %s\n" % (user, hashify(pswd))
+    )
 
     return contents + "\n\n[chttpd_auth]\nsecret = %s\n" % COMMON_SALT