Add installer for Mullvad VPN

.github/workflows/mullvad-vpn.yml

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+name: Mullvad VPN
+on:
+  push:
+    paths:
+      - programs/mullvad-vpn/install.bash
+      - .github/workflows/Mullvad VPN.yml
+  schedule:
+    # Not at the start of an hour or a day to decreate the chance of delay.
+    # See schedule documentation.
+    # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule
+    - cron: "17 5 * * *"
+
+  workflow_dispatch:
+
+jobs:
+  test_idempotent_installation:
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - "ubuntu:20.04"
+          - "ubuntu:22.04"
+
+    runs-on: ubuntu-latest
+    container:
+      image: "${{ matrix.os }}"
+
+    steps:
+
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Prepare Docker
+        run: ./scripts/prepare_docker.bash
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Install Mullvad VPN
+        shell: su norm --command "bash --login {0}"
+        run: programs/mullvad-vpn/install.bash
+
+      - name: Install Mullvad VPN again (test idempotency)
+        shell: su norm --command "bash --login {0}"
+        run: programs/mullvad-vpn/install.bash

programs/mullvad-vpn/install.bash

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+#!/bin/bash
+
+# Name: Mullvad VPN
+# https://mullvad.net/en/download/vpn/linux
+
+set -euxo pipefail
+
+cd "$(mktemp --dir)"
+
+sudo apt-get update
+
+# Installer dependencies
+sudo apt-get --assume-yes install \
+curl \
+lsb-release
+
+# Download the Mullvad signing key
+sudo curl -fsSLo /usr/share/keyrings/mullvad-keyring.asc https://repository.mullvad.net/deb/mullvad-keyring.asc
+
+arch=$( dpkg --print-architecture )
+release=$(lsb_release -cs)
+
+# Add the Mullvad repository server to apt
+echo "deb [signed-by=/usr/share/keyrings/mullvad-keyring.asc arch=${arch}] https://repository.mullvad.net/deb/stable ${release} main" | sudo tee /etc/apt/sources.list.d/mullvad.list
+
+# Install the package
+sudo apt update
+sudo apt install --assume-yes mullvad-vpn
+
+mullvad --version