GHA CI: Backport latest changes (#15361)

By having unified github workflows, the building cache would be utilized
more efficiently as the total cache size will be smaller (no more vcpkg
caching on macOS CI) and will stop thrashing the build cache (large
vcpkg cache evicts other smaller cache).

Relevant PRs:
#15321
#15340
#15342
#15355

.github/workflows/ci_ubuntu.yaml

@@ -0,0 +1,84 @@
+name: CI - Ubuntu
+on: [pull_request, push]
+
+jobs:
+  ci:
+    name: Build
+    runs-on: ubuntu-20.04
+
+    strategy:
+      matrix:
+        libt_version: ["v1.2.12"]
+        qbt_gui: ["GUI=ON", "GUI=OFF"]
+      fail-fast: false
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Setup ccache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ runner.os }}
+
+      - name: Install dependencies
+        run: |
+          sudo apt update
+          sudo apt install \
+            build-essential cmake git ninja-build pkg-config \
+            libssl-dev libgeoip-dev zlib1g-dev \
+            libboost-dev libboost-chrono-dev libboost-random-dev libboost-system-dev
+        # sudo apt install libqt5svg5-dev qtbase5-dev qttools5-dev # the Qt version in the standard repositories is too old...
+
+      # this will be installed under /opt/qt515. CMake will still find it automatically without additional hints
+      # to speed up the process, only the required components are installed rather than the full qt515-meta-full metapackage
+      - name: Install Qt
+        run: |
+          sudo add-apt-repository ppa:beineri/opt-qt-5.15.2-focal
+          sudo apt install \
+            qt515base qt515svg qt515tools
+
+      - name: Install libtorrent
+        run: |
+          git clone --branch ${{ matrix.libt_version }} --depth 1 https://github.com/arvidn/libtorrent.git
+          cd libtorrent
+          git submodule update --init --recursive
+          export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
+          cmake \
+            -B build \
+            -G "Ninja" \
+            -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+            -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
+            -Ddeprecated-functions=OFF \
+            --graphviz=cmake-build-dir/target_graph.dot
+          cmake --build build
+          sudo cmake --install build
+
+      - name: Build qBittorrent
+        run: |
+          export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
+          cmake \
+            -B build \
+            -G "Ninja" \
+            -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+            -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
+            -D${{ matrix.qbt_gui }} \
+            -DVERBOSE_CONFIGURE=ON \
+            --graphviz=build/target_graph.dot
+          cmake --build build
+
+      - name: Install qBittorrent
+        run: sudo cmake --install build
+
+      - name: Upload build artifacts
+        uses: actions/upload-artifact@v2
+        with:
+          name: qBittorrent-CI_ubuntu-20.04-x64_${{ matrix.qbt_gui }}_libtorrent-${{ matrix.libt_version }}
+          path: |
+            build/compile_commands.json
+            build/install_manifest.txt
+            build/target_graph.dot
+            build/qbittorrent
+            build/qbittorrent-nox
+            libtorrent/cmake-build-dir/compile_commands.json
+            libtorrent/cmake-build-dir/target_graph.dot