ci/linux: bundle an up to date version of libXft

This fixes the following error when you launch the app with an emoji
font installed (e.g. Noto Emoji):

X Error of failed request:  BadLength (poly request too large or internal Xlib length error)
  Major opcode of failed request:  139 (RENDER)
  Minor opcode of failed request:  20 (RenderAddGlyphs)
  Serial number of failed request:  277
  Current serial number in output stream:  300

This has been fixed[1] in libXft 2.3.5[2], but unfortunately Ubuntu
20.04 (which we currently use in the Linux CI workflow), still has
version 2.3.3, so we have to build it ourselves.

[1] https://gitlab.freedesktop.org/xorg/lib/libxft/-/merge_requests/12
[2] https://lists.freedesktop.org/archives/xorg-announce/2022-September/003209.html

.github/workflows/ci.yml

@@ -108,10 +108,22 @@ jobs:
           source ./env/bin/activate
           python3 -m pip install pyinstaller
 
+      # NOTE: Remove this step if/once libxft gets updated to 2.3.5 or newer on Ubuntu 20.04, which currently has 2.3.3.
+      - name: Build a recent version of libXft
+        run: |
+          mkdir -p /tmp/libXft
+          cd /tmp/libXft
+          curl -L https://xorg.freedesktop.org/releases/individual/lib/libXft-2.3.8.tar.xz -o libXft.tar.xz
+          tar xvf libXft.tar.xz
+          cd libXft-*
+          ./configure --prefix=/tmp/libXft --sysconfdir=/etc --disable-static
+          make
+          make install-strip
+
       - name: Create portable executable
         run: |
           source ./env/bin/activate
-          xvfb-run --auto-servernum pyinstaller build.spec
+          LD_LIBRARY_PATH=/tmp/libXft/lib xvfb-run --auto-servernum pyinstaller build.spec
 
       - name: Show PyInstaller warnings
         run: |