GHA CI: Remove OS variable from build matrix

It is meaningless to build on multiple linux versions as we only depend
on library versions, not OS versions.
Also remove redundant "shell default" section.

.github/workflows/ci.yaml

@@ -14,20 +14,14 @@ env:
 jobs:
   ubuntu:
     name: Ubuntu
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix:
-        os: [ubuntu-20.04]
         qbt_gui: ["GUI=ON", "GUI=OFF"]
         libt_version: ["v2.0.4", "v1.2.14"]
       fail-fast: false
 
-    runs-on: ${{ matrix.os }}
-
-    defaults:
-      run:
-        shell: bash
-
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
@@ -82,7 +76,7 @@ jobs:
       - name: Upload build artifacts
         uses: actions/upload-artifact@v2
         with:
-          name: qBittorrent-CI_${{ matrix.os }}-x64_${{ matrix.qbt_gui }}_libtorrent-${{ matrix.libt_version }}
+          name: qBittorrent-CI_ubuntu-20.04-x64_${{ matrix.qbt_gui }}_libtorrent-${{ matrix.libt_version }}
           path: |
             build/compile_commands.json
             build/install_manifest.txt