DOC: Remove Qt var from main linux build instructions

It's only needed on ubuntu 20.04 pretty much so it should not be there
in the main instruction.

Author: ebrahimebrahim

{{cookiecutter.project_name}}/BUILD_LINUX.md

@@ -20,19 +20,21 @@ Install the prerequisites as described in the [Slicer documentation for building
 
 Note: The build process can take hours.
 
-Assuming that Qt 5.15.2 was installed to `/opt/qt/` (if not then adjust the command accordingly):
+Build:
 
 ```sh
 cmake \
   -DCMAKE_BUILD_TYPE:STRING=Release \
   -DQt5_DIR:PATH=/opt/qt/5.15.2/gcc_64/lib/cmake/Qt5 \
   -S {{cookiecutter.github_project}} \
   -B {{cookiecutter.github_project}}-SuperBuild-Release
-
+cd {{cookiecutter.github_project}}-SuperBuild-Release
 make -j<N>
 ```
 
 where `<N>` is the number of parallel builds. As a rule of thumb, many use the `number of CPU threads - 1` as the number of parallel builds.
+On Ubuntu 20.04, the default Qt5 packages are too old and so the Slicer documentation linked above should have suggested a method of installing Qt 5.15.2;
+if you installed it to `/opt/qt`, for example, then an extra option like `-DQt5_DIR:PATH=/opt/qt/5.15.2/gcc_64/lib/cmake/Qt5` would be needed in the `cmake` command above.
 
 Once the application is built, there will be an _inner build_ inside the _superbuild_ folder, located at `{{cookiecutter.app_name}}-SuperBuild-Release/Slicer-build`. The application executable is contained in this _inner build_ folder.