Add basic FreeBSD CI using Cirrus CI (#923)

This currently only runs unit tests and not the compliance tests.

Cirrus CI is the only CI provider I'm aware of which supports FreeBSD
out of the box. It does have the disadvantage though that on the free
tier it can take several minutes before a CI job actually starts.

---------

Co-authored-by: Marc R. Schoolderman <[email protected]>

Author: bjorn3

.cirrus.yml

@@ -0,0 +1,13 @@
+task:
+  name: Cirrus CI / freebsd unit tests
+  freebsd_instance:
+    image_family: freebsd-14-2
+    memory: 2GB
+  setup_rust_script:
+    - pkg install -y git-tiny
+    - curl https://sh.rustup.rs -sSf --output rustup.sh
+    - sh rustup.sh -y --profile=minimal
+  test_script:
+    - . $HOME/.cargo/env
+    # We skip a couple of tests which fail when running as root.
+    - cargo test --workspace --all-features --all-targets --release -- --skip group_as_non_root --skip test_secure_open_cookie_file