ENH: Update GitHub workflow to test build against Qt 5.12.8

This update to the GitHub workflow enables testing builds against Qt 5.12.8
on Ubuntu 20.04. Notably, the build excludes VTK due to unavailability of
VTK 9 packages on Ubuntu 20.04.

To support building ctkDICOMCore on Ubuntu 20.04, DCMTK is now built from source
using the CTK SuperBuild. This decision stems from the absence of DCMTK 3.6.6, as
indicated by the DCMTK package version 3.6.4 available[^1] via apt-get. The need for
building DCMTK from source arises from the utilization of the function
`DcmSCP::acceptAssociations`, introduced in DCMTK 3.6.6
through DCMTK/dcmtk@c3d4c8e09 (Added alternative service provision API to DcmSCP).

Additionally, to ensure completion of the independent GitHub CI build workflows
testing against Qt 5.15.2 and Qt 5.12.8, the "fail-fast" option is set to false.

[^1]: https://packages.ubuntu.com/focal/dcmtk

.github/workflows/ci.yml

@@ -16,7 +16,11 @@ permissions:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.runs-on }}
+    strategy:
+      fail-fast: false
+      matrix:
+        runs-on: ["ubuntu-latest", "ubuntu-20.04"]
     steps:
       - uses: actions/checkout@9bb56186c3b09b4f86b1c65136769dd318469633 # v4.1.2
 
@@ -32,12 +36,6 @@ jobs:
       - name: Retrieve new lists of APT packages
         run: sudo apt-get update
 
-      - name: Install DCMTK
-        run: |
-          sudo apt-get install -y \
-            dcmtk \
-            libdcmtk-dev
-
       - name: Install Qt
         run: |
           sudo apt-get install -y \
@@ -53,6 +51,7 @@ jobs:
             libxt-dev
 
       - name: Install VTK
+        if: matrix.runs-on == 'ubuntu-latest'
         run: |
           sudo apt-get install -y \
             libvtk9-dev \
@@ -63,15 +62,14 @@ jobs:
           cmake \
             -DCTK_QT_VERSION:STRING=5 \
             -DCTK_ENABLE_Widgets:BOOL=ON \
-            -DCTK_USE_SYSTEM_VTK:BOOL=ON \
-            -DCTK_LIB_Visualization/VTK/Widgets:BOOL=ON \
-            -DCTK_LIB_Visualization/VTK/Widgets_USE_TRANSFER_FUNCTION_CHARTS:BOOL=ON \
-            -DCTK_USE_SYSTEM_DCMTK:BOOL=ON \
+            -DCTK_USE_SYSTEM_VTK:BOOL=$CTK_USE_VTK \
+            -DCTK_LIB_Visualization/VTK/Widgets:BOOL=$CTK_USE_VTK \
+            -DCTK_LIB_Visualization/VTK/Widgets_USE_TRANSFER_FUNCTION_CHARTS:BOOL=$CTK_USE_VTK \
             -DCTK_APP_ctkDICOM:BOOL=ON \
             -DCTK_LIB_DICOM/Core:BOOL=ON \
             -DCTK_LIB_DICOM/Widgets:BOOL=ON \
             -DCTK_LIB_Scripting/Python/Core:BOOL=ON \
-            -DCTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK:BOOL=ON \
+            -DCTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK:BOOL=$CTK_USE_VTK \
             -DCTK_LIB_Scripting/Python/Core_PYTHONQT_WRAP_QTCORE:BOOL=ON \
             -DCTK_LIB_Scripting/Python/Core_PYTHONQT_WRAP_QTGUI:BOOL=ON \
             -DCTK_LIB_Scripting/Python/Core_PYTHONQT_WRAP_QTUITOOLS:BOOL=ON \
@@ -82,6 +80,8 @@ jobs:
             -DCTK_ENABLE_Python_Wrapping:BOOL=ON \
             -B CTK-build \
             -S .
+        env:
+          CTK_USE_VTK: "${{ matrix.runs-on == 'ubuntu-latest' && 'ON' || 'OFF' }}"
 
       - name: Display Qt5_DIR
         run: |