chore: allow RUST_LOG to be overridden

This just uses the `export ... ?= ...` approach to use the value from
the parent environment if set, or a default if not.

Author: Chris Connelly

Makefile

@@ -6,6 +6,7 @@ export PGUSER ?= postgres
 export PGPASSWORD ?= password
 export PGDATABASE ?= sample
 export DATABASE_URL ?= postgres://$(PGUSER):$(PGPASSWORD)@$(PGHOST):$(PGPORT)/$(PGDATABASE)
+export RUST_LOG ?= rust_graphql_sample=debug,tower_http=debug
 
 prepare-db: start-db
 	@sqlx database create
@@ -18,4 +19,4 @@ stop-db:
 	@scripts/stop-db.sh
 
 run: start-db prepare-db
-	@RUST_LOG=rust_graphql_sample=debug,tower_http=debug cargo run
+	@cargo run