ci: update FreeBSD workflow values to 15.1 (#66)

* ci: use FreeBSD 15.1 in workflow

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: asomers

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
         include:
           - freebsd_version: "14.4"
             rust_version: "stable"
-          - freebsd_version: "15.0"
+          - freebsd_version: "15.1"
             rust_version: "nightly"
     name: FreeBSD ${{ matrix.freebsd_version }} ${{ matrix.rust_version }}
     steps: