Bump Qt 5.12.10 to 5.12.12 (#3809)

Normalize cache method between build and test workflow
Normalize Qt install method between build and test workflow
Add explicit Qt version in test workflow
Allow builds to be triggered through workflow dispatch https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

.github/workflows/test.yml

@@ -3,6 +3,7 @@ name: Test
 
 on:
   pull_request:
+  workflow_dispatch:
 
 env:
   TWITCH_PUBSUB_SERVER_IMAGE: ghcr.io/chatterino/twitch-pubsub-server-test:v1.0.3
@@ -17,6 +18,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04]
+        qt-version: [5.15.2]
       fail-fast: false
 
     steps:
@@ -28,20 +30,16 @@ jobs:
         id: cache-qt
         uses: actions/cache@v3
         with:
-          path: ../Qt
-          key: ${{ runner.os }}-QtCache-20201005
+          path: "${{ github.workspace }}/qt/"
+          key: ${{ runner.os }}-QtCache-${{ matrix.qt-version }}
 
       # LINUX
-      - name: Install p7zip (Ubuntu)
-        if: startsWith(matrix.os, 'ubuntu')
-        run: sudo apt-get update && sudo apt-get -y install p7zip-full
-
       - name: Install Qt
         uses: jurplel/install-qt-action@v2
         with:
-          aqtversion: '==1.1.1'
           cached: ${{ steps.cache-qt.outputs.cache-hit }}
-          extra: --external 7z
+          version: ${{ matrix.qt-version }}
+          dir: "${{ github.workspace }}/qt/"
 
       # LINUX
       - name: Install dependencies (Ubuntu)