Utils: Replace FilePath::onDevice() by new FilePath::withMappedPath()

Basically a.onDevice(b) == b.withNewMappedPath(a), matching the order
of b.withNewPath(a).

Whether the (curretly docker-specific) path mapping is useful /there/, and
whether some of the calls are needed at all is dubious. I added some
FIXME and changed a few cases directly.

Change-Id: I7514736ce922f632f1f737bc496f6783389a42b6
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/cmakeprojectmanager/cmakebuildstep.cpp

@@ -383,7 +383,7 @@ CommandLine CMakeBuildStep::cmakeCommand() const
     if (buildConfiguration())
         buildDirectory = buildConfiguration()->buildDirectory();
 
-    cmd.addArgs({"--build", buildDirectory.onDevice(cmd.executable()).path()});
+    cmd.addArgs({"--build", cmd.executable().withNewMappedPath(buildDirectory).path()});
 
     cmd.addArg("--target");
     cmd.addArgs(Utils::transform(m_buildTargets, [this](const QString &s) {