Revert "chore: disable warnings in old CI (#4691)"

This reverts commit b24df49a9d so we can
merge the CHANGELOG changes in #4691 into master.

.cirrus.yml

@@ -1,5 +1,7 @@
 freebsd_instance:
   image: freebsd-12-2-release-amd64
+env:
+  RUSTFLAGS: -D warnings
 
 # Test FreeBSD in a full VM on cirrus-ci.com.  Test the i686 target too, in the
 # same VM.  The binary will be built in 32-bit mode, but will execute on a
@@ -23,7 +25,7 @@ task:
   name: FreeBSD docs
   env:
     RUSTFLAGS: --cfg docsrs
-    RUSTDOCFLAGS: --cfg docsrs
+    RUSTDOCFLAGS: --cfg docsrs -Dwarnings
   setup_script:
     - pkg install -y bash curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh