chore(gui-client): reuse release-version constant (#8074)

This constant already defines the same string:
36f5eee99d/rust/gui-client/src-common/src/lib.rs (L19)

rust/gui-client/src-tauri/src/client.rs

@@ -63,7 +63,7 @@ pub(crate) fn run() -> Result<()> {
             let mut telemetry = telemetry::Telemetry::default();
             telemetry.start(
                 settings.api_url.as_ref(),
-                concat!("gui-client@", env!("CARGO_PKG_VERSION")),
+                firezone_gui_client_common::RELEASE,
                 telemetry::GUI_DSN,
             );
             // Don't fix the log filter for smoke tests