CMakeProjectManager: Use buildDirectory instead of "."

When issuing cmake --build "." as an user I don't get to see which build
directory is being actually built.

To make things worse in project settings the build directory field is
disabled and cannot be copied.

Change-Id: Idf862ba1df68cde55e9785c934f67393da6c1616
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.cpp

@@ -379,7 +379,10 @@ CommandLine CMakeBuildStep::cmakeCommand() const
     CMakeTool *tool = CMakeKitAspect::cmakeTool(kit());
 
     Utils::CommandLine cmd(tool ? tool->cmakeExecutable() : Utils::FilePath(), {});
-    cmd.addArgs({"--build", "."});
+    QString buildDirectory = ".";
+    if (buildConfiguration())
+        buildDirectory = buildConfiguration()->buildDirectory().toString();
+    cmd.addArgs({"--build", buildDirectory});
 
     cmd.addArg("--target");
     cmd.addArgs(Utils::transform(m_buildTargets, [this](const QString &s) {