Pin the nightly version of rust

CI was failing on the latest nightly versions of rust due to
rust-lang/packed_simd#288

This commit pins the version of nightly rust to an older version to
avoid this issue.

.gitlab-ci.yml

@@ -1,3 +1,5 @@
+# We've pinned the nightly toolchain to nightly-2020-09-15 until https://github.com/rust-lang/packed_simd/issues/288 is fixed.
+
 image: "rust:latest"
 
 variables:
@@ -46,12 +48,12 @@ test:stable:
 test:without-stable-feature:
   extends: .test
   variables:
-    toolchain: nightly
+    toolchain: nightly-2020-09-15
     features: curve25519-dalek serde
 
 test:nightly:
   extends: .test
   variables:
-    toolchain: nightly
+    toolchain: nightly-2020-09-15
     features: curve25519-dalek serde nightly