chore: Retry freebsd tests 3 times.

Also, use `cmake --build` instead of manually calling `gmake`. This
allows us to maybe later use `ninja` instead of `gmake` without changing
this build invocation.

.github/scripts/cmake-freebsd

@@ -10,7 +10,10 @@ cd .. # /work
 
 # === Get VM ready to build the code ===
 
-gunzip "$IMAGE_NAME.gz"
+# Unpack image only if it's compressed.
+if [ -f "$IMAGE_NAME.gz" ]; then
+  gunzip "$IMAGE_NAME.gz"
+fi
 
 mv c-toxcore /
 
@@ -48,8 +51,8 @@ RUN "cmake -B_build -Hc-toxcore \
   -DAUTOTEST=ON"
 
 # We created the VM with the same number of cores as the host, so the host-ran `nproc` here is fine.
-RUN 'gmake "-j$NPROC" -k install -C_build'
-RUN 'gmake "-j$NPROC" test ARGS="-j50" -C_build || true'
+RUN 'cmake --build _build --parallel "$NPROC" --target install -- -k'
+RUN 'cd _build && ctest -j50 --output-on-failure --rerun-failed --repeat until-pass:3 --timeout 90 || true'
 
 # Gracefully shut down the VM.
 stop_vm