COMP: Fix ctkDICOMCore build in Ubuntu 20.04 CI ensuring use of lates…

…t DCMTK

Since DCMTK package installed using apt-get is 3.6.4 (see [1]) and
the function `DcmSCP::acceptAssociations` was introduced in DCMTK 3.6.6
through DCMTK/dcmtk@c3d4c8e09 (Added alternative service provision API to DcmSCP.),
switch to building DCMTK from source.

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

.github/workflows/ci.yml

@@ -37,12 +37,6 @@ 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 \
@@ -72,7 +66,6 @@ jobs:
             -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_USE_SYSTEM_DCMTK:BOOL=ON \
             -DCTK_APP_ctkDICOM:BOOL=ON \
             -DCTK_LIB_DICOM/Core:BOOL=ON \
             -DCTK_LIB_DICOM/Widgets:BOOL=ON \