Re-intrudice FreeBSD 12 in the Cirrus CI config.

FreeBSD 12 was origally removed in #517 due to problems with `curl`,
however those were fixed by removing the `pkg install -y curl` in #643,
so we can now re-enable FreeBSD 12 testing.

.cirrus.yml

@@ -13,3 +13,16 @@ task:
   test_script:
     - . $HOME/.cargo/env
     - cargo test --workspace --features=all-apis
+
+task:
+  name: stable x86_64-unknown-freebsd-12
+  freebsd_instance:
+    image_family: freebsd-12-1
+  setup_script:
+    - curl https://sh.rustup.rs -sSf --output rustup.sh
+    - sh rustup.sh --default-toolchain stable -y --profile=minimal
+    - . $HOME/.cargo/env
+    - rustup default stable
+  test_script:
+    - . $HOME/.cargo/env
+    - cargo test --workspace --features=all-apis