fix(setup): skip apt for acarsdec, build from source directly (#183)

acarsdec is not available in apt repos, so the apt_install attempt
always failed with a confusing error message before falling through
to the source build. Skip the apt attempt and go straight to compiling
from source on Linux.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

setup.sh

@@ -1485,9 +1485,10 @@ install_tool_acarsdec() {
     fi
   else
     if ! cmd_exists acarsdec; then
-      apt_install acarsdec || true
+      install_acarsdec_from_source_debian
+    else
+      ok "acarsdec already installed"
     fi
-    cmd_exists acarsdec || install_acarsdec_from_source_debian
   fi
 }