Reenable CI on FreeBSD 15

The freebsd-15-0-current-amd64-ufs-20241017 GCE image fixes the libmd.so
problem, and once again works with Cirrus CI.

Fixes rust-lang#3967

(backport <rust-lang#3988>)
(cherry picked from commit 86d3d1e)

Author: asomers

.cirrus.yml

@@ -7,13 +7,9 @@ task:
     - name: nightly freebsd-14
       freebsd_instance:
         image: freebsd-14-0-release-amd64-ufs
-    # Temporarily disable CI on FreeBSD 15.0-CURRENT until the libmd solib
-    # fallout is cleaned up.
-    # FIXME https://github.com/rust-lang/libc/issues/3967
-    # https://github.com/rust-lang/libc/issues/3967
-    #- name: nightly freebsd-15
-    # freebsd_instance:
-    # image_family: freebsd-15-0-snap
+    - name: nightly freebsd-15
+      freebsd_instance:
+       image_family: freebsd-15-0-snap
   setup_script:
     - pkg install -y libnghttp2 curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh