ENH: CI: Disable MIT_SHM X11 extension

Set environment variable QT_X11_NO_MITSHM=1 to disable MIT_SHM X11
extension. This avoids CI errors like:

    X Error: BadAccess (attempt to access private resource denied) 10
      Extension:    130 (MIT-SHM)
      Minor opcode: 1 (X_ShmAttach)
      Resource id:  0x11b

CMake/CircleCI/CircleCI_CTK_Docker.cmake

@@ -67,6 +67,8 @@ set( CMAKE_BUILD_TYPE "${SITE_BUILD_TYPE}")
 set( BUILD_TESTING ON )
 set( CTK_BUILD_EXAMPLES OFF )
 
+# Disable MIT_SHM X11 extension
+set( ENV{QT_X11_NO_MITSHM} 1 )
 
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