fix(tvix/glue/tvix_store_io): use same case for progress messages

"Fetching" was uppercase, "building" was lowercase.
Let's make this consistent.

Change-Id: I11c16f1a7d2057ada4d057e553a4ceaa59597f26
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11796
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/glue/src/tvix_store_io.rs

@@ -183,7 +183,7 @@ impl TvixStoreIO {
                                 }
                             }
                         };
-                        span.pb_set_message(&format!("🔨building {}", &store_path));
+                        span.pb_set_message(&format!("🔨Building {}", &store_path));
 
                         // derivation_to_build_request needs castore nodes for all inputs.
                         // Provide them, which means, here is where we recursively build