CI: Add FreeBSD build tests for staged commits (#1948)

Use a GitHub-hosted VM to create FreeBSD test environment.

We have not enabled ccache optimization for FreeBSD tests because we
do not know how to copy updated ccache files from VM back into runner.

.github/workflows/slow.yaml

@@ -134,3 +134,50 @@ jobs:
         with:
           name: build-logs-macos
           path: btlayer-*.log
+
+  freebsd:
+    strategy:
+      matrix:
+        osversion:
+          - 14.1
+          - 13.3
+
+    runs-on: ubuntu-22.04
+    name: freebsd(${{ matrix.osversion }})
+
+    steps:
+      - name: Checkout Sources
+        uses: actions/checkout@v4
+
+      - name: Run test-builds
+        id: test-builds
+        uses: vmactions/freebsd-vm@v1
+        with:
+          usesh: true
+          release: ${{ matrix.osversion }}
+          prepare: |
+            pkg update
+            pkg install -y \
+              autoconf \
+              autoconf-archive \
+              automake \
+              bash \
+              cppunit \
+              gmake \
+              libltdl \
+              libtool \
+              m4 \
+              nettle \
+              pkgconf \
+              translate-toolkit
+
+          run: |
+            export MAKE=gmake
+            ./test-builds.sh
+
+      - name: Publish build logs
+        if: success() || failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: build-logs-freebsd-${{ matrix.osversion }}
+          path: btlayer-*.log