refactor(tvix/glue): make user-agent more granular

Use the crate name in the user-agent, similar to tvix-[ca]store.

Change-Id: I10527fb1f29006dbfd8630d8bb1f00d7905efdd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12851
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Vladimir Kryachko <v.kryachko@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/glue/src/lib.rs

@@ -7,6 +7,9 @@ pub mod tvix_store_io;
 
 mod fetchurl;
 
+// Used as user agent in various HTTP Clients
+const USER_AGENT: &str = concat!(env!("CARGO_PKG_NAME"), "/", env!("CARGO_PKG_VERSION"));
+
 #[cfg(test)]
 mod tests;