Fix FreeBSD CI failure

This also removes the workaround for the fixed rustup bug.

Author: taiki-e

.cirrus.yml

@@ -1,13 +1,8 @@
-env:
-  # Temporary fix for https://github.com/rust-lang/rustup/issues/2774.
-  RUSTUP_IO_THREADS: "1"
-
 task:
   name: x86_64-unknown-freebsd
   freebsd_instance:
-    image: freebsd-12-2-release-amd64
+    image_family: freebsd-12-4
   setup_script:
-    - pkg install -y curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh --default-toolchain nightly -y
     - . $HOME/.cargo/env