[ci] ubuntu18: no c++17 (#12196)

* [ci] Add trailing newline.

* [ci] Switch to c++14 without root7: missing charconv.

.github/workflows/rootci-installers/buildconfig/ubuntu18.txt

 CMAKE_BUILD_TYPE=RelWithDebInfo
-CMAKE_CXX_STANDARD=17
+CMAKE_CXX_STANDARD=14
 LLVM_ENABLE_ASSERTIONS=On
 alien=Off
 arrow=Off
@@ -78,6 +78,7 @@ pythia8=On
 qt5web=On
 qt6web=Off
 r=On
+root7=Off
 roofit=On
 roofit_multiprocess=On
 roottest=ON