ci/linux: use git version of appimage-builder

appimage-builder is a tool that is not officially part of the AppImage
ecosystem, and lately its maintainer hasn't been able to keep up with
the latest AppImage ideas, most notably:

- The move to a new statically-linked runtime: Before this, all AppImages
using the old runtime, would depend on libfuse2 to be installed on the
host system just to be able to run out of the box. This wasn't much of
a problem back then, because libfuse2 was ubiquitous. However, nowadays,
some have distros don't even have it in their official repositories.
- The removal of the xz compression algorithm (used by default), and the
addition of Zstd.

Luckily, the problems above have been fixed in the git version of
appimage-builder, but a stable release is yet to be made, so we'll have
to use the git version, because other AppImage-making tools are not as
nice and convenient as appimage-builder.

.github/workflows/ci.yml

@@ -200,18 +200,17 @@ jobs:
           sudo apt update
           sudo apt install libgirepository1.0-dev gir1.2-ayatanaappindicator3-0.1 libayatana-appindicator3-1 python3-testresources
 
-      - name: Download AppImage Builder
+      - name: Install appimage-builder
         run: |
-          curl -L https://github.com/AppImageCrafters/appimage-builder/releases/download/v1.1.0/appimage-builder-1.1.0-x86_64.AppImage -o appimage-builder
-          chmod +x appimage-builder
+          # TODO: Switch to a stable release if/when a version newer than 1.1.0 is released.
+          python3 -m pip install git+https://github.com/AppImageCrafters/appimage-builder.git@e995e8edcc227d14524cf39f9824c238f9435a22
 
       - name: Create AppImage
         env:
-          APPIMAGE_EXTRACT_AND_RUN: 1
           APP_VERSION: ${{steps.vars.outputs.app_version}}.${{steps.vars.outputs.sha_short}}
           PYTHON_VERSION: ${{env.PYTHON_VERSION}}
         run: |
-          ./appimage-builder --recipe appimage/AppImageBuilder.yml
+          appimage-builder --recipe appimage/AppImageBuilder.yml
 
       - name: Create release folder
         run: |