QbsProjectManager: Avoid allocations when checking environment variables

Use qEnvironmentVariableIsEmpty instead of qgetenv(...).isEmpty().
Found by clazy.

Change-Id: I62054e68c3b51c69be4ce6605ed6aef20633c9a9
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qbsprojectmanager/qbsbuildstep.cpp

@@ -497,7 +497,7 @@ void QbsBuildStep::build()
     options.setChangedFiles(m_changedFiles);
     options.setFilesToConsider(m_changedFiles);
     options.setActiveFileTags(m_activeFileTags);
-    options.setLogElapsedTime(!qgetenv(Constants::QBS_PROFILING_ENV).isEmpty());
+    options.setLogElapsedTime(!qEnvironmentVariableIsEmpty(Constants::QBS_PROFILING_ENV));
 
     QString error;
     m_job = qbsProject()->build(options, m_products, error);

src/plugins/qbsprojectmanager/qbsprojectparser.cpp

@@ -108,7 +108,7 @@ void QbsProjectParser::parse(const QVariantMap &config, const Environment &env,
     params.setLibexecPath(libExecDirectory());
     params.setProductErrorMode(qbs::ErrorHandlingMode::Relaxed);
     params.setPropertyCheckingMode(qbs::ErrorHandlingMode::Relaxed);
-    params.setLogElapsedTime(!qgetenv(Constants::QBS_PROFILING_ENV).isEmpty());
+    params.setLogElapsedTime(!qEnvironmentVariableIsEmpty(Constants::QBS_PROFILING_ENV));
 
     m_qbsSetupProjectJob = m_project.setupProject(params, QbsManager::logSink(), 0);