Move GIT_VERSION injection later in build chain to reduce build times

src/model/version.rs

@@ -0,0 +1,7 @@
+pub fn garage_version() -> &'static str {
+	option_env!("GIT_VERSION").unwrap_or(git_version::git_version!(
+		prefix = "git:",
+		cargo_prefix = "cargo:",
+		fallback = "unknown"
+	))
+}