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/build.yml

@@ -6,6 +6,7 @@ on:
     branches:
       - master
   pull_request:
+  workflow_dispatch:
 
 concurrency: 
   group: build-${{ github.ref }}
@@ -17,7 +18,7 @@ jobs:
     strategy:
       matrix:
         os: [windows-latest, ubuntu-latest, macos-latest]
-        qt-version: [5.15.2, 5.12.10]
+        qt-version: [5.15.2, 5.12.12]
         build-system: [qmake, cmake]
         pch: [true]
         include:
@@ -47,10 +48,6 @@ jobs:
           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: