Always set an environment for QtcProcess

Since e755094480 we have a global option
in Qt Creator to set the (base) environment for any tools run by Qt
Creator. So it is expected that any tool actually uses
Environment::systemEnvironment() as a base, which is the system
environment modified by the global option.

QtcProcess already had a fallback to Environment::systemEnvironment for
resolving environment variables in Qt Creator macros in the command
line. Also use it as a fallback for the actual environment for the
process.

Latest occurrence where the global environment setting was ignored was
for starting language servers by our LSP client.

Change-Id: Id1a4141326d8ef3239b37d7f3daf21928605ca92
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/libs/utils/qtcprocess.cpp

@@ -350,11 +350,10 @@ void QtcProcess::start()
             qWarning("QtcProcess::start: Empty environment set when running '%s'.",
                      qPrintable(d->m_commandLine.executable().toString()));
         env = d->m_environment;
-
-        d->m_process->setProcessEnvironment(env.toProcessEnvironment());
     } else {
         env = Environment::systemEnvironment();
     }
+    d->m_process->setProcessEnvironment(env.toProcessEnvironment());
 
     const QString workDir = d->m_workingDirectory.path();
     d->m_process->setWorkingDirectory(workDir);