gitlab-ci: switch archlinux job to virtualenv

https://gitlab.com/fdroid/fdroidserver/-/jobs/4466370098

$ pip install -e .[test]
error: externally-managed-environment
× This environment is externally managed
╰─> To install Python packages system-wide, try 'pacman -S
    python-xyz', where xyz is the package you are trying to
    install.

    If you wish to install a non-Arch-packaged Python package,
    create a virtual environment using 'python -m venv path/to/venv'.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip.

    If you wish to install a non-Arch packaged Python application,
    it may be easiest to use 'pipx install xyz', which will manage a
    virtual environment for you. Make sure you have python-pipx
    installed via pacman.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.

.gitlab-ci.yml

@@ -170,6 +170,8 @@ arch_pip_install:
     - master@fdroid/fdroidserver
   script:
     - pacman --sync --sysupgrade --refresh --noconfirm gcc git grep python-pip python-virtualenv python-wheel tar
+    - python -m venv venv
+    - source venv/bin/activate
     - pip install -e .[test]
     - fdroid
     - fdroid readmeta