Revert "stick to 8.17 for nightly testing for now"

This reverts commit 4243284b.
Robbert's previous commit should make this 8.18-compatible.

.gitlab-ci.yml

@@ -6,7 +6,6 @@ stages:
 variables:
   CPU_CORES: "10"
   OCAML: "ocaml-variants.4.14.0+options ocaml-option-flambda"
-  NIGHTLY_COQ: "8.17.dev" # 8.18.dev causes an anomaly
 
 .template: &template
   stage: build