Switch CI back to nightly.

Now that they have merged
rust-lang/rust#104416 ,
our CI should pass again with the regular nightly image.

Rather than remove the pinned image entirely, I'm commenting it out,
with a comment explaining how to pin an image the next time we need
to do so.

Closes #633.

Author: nmathewson

.gitlab-ci.yml

@@ -91,9 +91,13 @@ rust-latest-async-std-rustls:
 
 rust-nightly:
   stage: test
-  #image: rustlang/rust:nightly
-  # Temporary workarounds for arti#633.
-  image: rustlang/rust@sha256:415b7c22ab4a8a3ec3efc9cc8d7b018964f0c6757fff27bbd110e0ed92566321
+  image: rustlang/rust:nightly
+  # In case there is a bug in rust:nightly, you can instead pin an older
+  # version of the Docker image until that bug is fixed.  To find the
+  # SHA256 ID of the last working version of nightly, look at the logs
+  # from the last successful CI run.  Here is an example of how to do so:
+  #
+  # image: rustlang/rust@sha256:415b7c22ab4a8a3ec3efc9cc8d7b018964f0c6757fff27bbd110e0ed92566321
   allow_failure: true
   script:
     - rustup show