CI: force pkg metadata refresh on FreeBSD

The FreeBSD 15.0 CMake job occasionally fails at package install when
the implicit pkg update during `pkg install` hits a truncated
signature fingerprint fetch. pkg prints "repository is up to date"
but leaves the on-disk index half-written, and the subsequent install
can't read it. `pkg update -f` forces a clean re-fetch regardless of
cached state.

Author: sgerbino

.github/workflows/ci.yml

@@ -437,6 +437,7 @@ jobs:
           release: ${{ matrix.version }}
           usesh: true
           prepare: |
+            pkg update -f
             pkg install -y cmake
           run: |
             set -xe