CI: prepare for nightly std++ builds

.gitlab-ci.yml

@@ -62,3 +62,12 @@ build-coq.8.6.1:
   <<: *template
   variables:
     OPAM_PINS: "coq version 8.6.1   coq-mathcomp-ssreflect version 1.6.4"
+
+build-stdpp.dev:
+  <<: *template
+  variables:
+    OPAM_PINS: "coq version 8.7.1   coq-mathcomp-ssreflect version 1.6.4   coq-stdpp.dev git https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp/#$STDPP_REV"
+  except:
+  only:
+  - triggers
+  - schedules