osbase/build: replace label_keyed_string_dict with string_keyed_label_dict

Before bazel 7.4 the string_keyed_label_dict attribute type wasn't available. fsspec_core_impl was using a label_keyed_string_dict which is structurally wrong but there was no alternative for it. This replaces that usage.

Change-Id: I36c02c84e6aa2557cd2beb09c07d3ceca501553d
Reviewed-on: https://review.monogon.dev/c/monogon/+/3853
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/installer/test/testos/BUILD.bazel b/metropolis/installer/test/testos/BUILD.bazel
index c41fbee..c7d58f0 100644
--- a/metropolis/installer/test/testos/BUILD.bazel
+++ b/metropolis/installer/test/testos/BUILD.bazel
@@ -6,7 +6,7 @@
 erofs_image(
     name = "rootfs",
     files = {
-        ":testos": "/init",
+        "/init": ":testos",
     },
 )