ci: Fixed build script for Ubuntu build (2)

scripts/build_ubuntu.sh

@@ -1,12 +1,22 @@
 #!/bin/sh
-#
+# Build LASP on Arch Linux, assuming we start in a clean docker container.
+# After that, test stuff.
+# Assumptions
+# - CWD is root of lasp repository
+# - Nothing is installed
+
+# Stop on first error
 set -e
+# Update cache
 apt update
-apt install -y python3-virtualenv libopenblas-dev python3-pip libusb-1.0-0-dev libpulse-dev python3-build
+# Install requirements
+apt install -y python3-virtualenv libopenblas-dev python3-pip libfftw3-dev libusb-1.0-0-dev libpulse-dev python3-build
+# Build
 git submodule update --init --recursive
 python3 -m build
 python3 -m venv .venv
 source .venv/bin/activate
 pip install pytest
 pip install dist/lasp-*linux_x86_64.whl
+# Test
 pytest