cargo: Moved tempfile crate to dev deps
Don't require the `tempfile` crate unless building in dev mode (such as
when running tests).
@@ -16,5 +16,6 @@ slog = { version = "2.5.2", features = ["dynamic-keys", "max_level_trace", "rele
slog-json = "2.3.0"
slog-async = "2.3.0"
slog-scope = "4.1.2"
-# for testing
+
+[dev-dependencies]
tempfile = "3.1.0"
