CI: add note to keep job name in sync with bors workflow

This was pointed out by tschwinge in a review [0]. Add a note to avoid
that error happening in future.

[0] Rust-GCC#3262 (comment)

ChangeLog:
	* .github/workflows/ccpp.yml: Add note to keep in sync with
	.github/bors.toml.

.github/workflows/ccpp.yml

@@ -266,6 +266,7 @@ jobs:
               exit 0; \
             fi
 
+  # This job name must be kept in sync with .github/bors.toml.
   build-and-check-gcc-5:
 
     runs-on: ubuntu-22.04