WebAssembly: (Win) Use python.exe from EMSDK_PYTHON instead from Path

This affects Windows only. Instead of searching python.exe in the Path,
use the executable name which is stored in the "EMSDK_PYTHON"
environment variable (which is set by emsdk_env).

emsdk_env also prepends entries to Path, but not the one pointing to the
emsdk-shipped python interpreter. I believe that earlier versions of
emsdk_env used to add the python location to Path, else this is an old
bug.

Fixes: QTCREATORBUG-25897
Change-Id: I9b5bc43f865d19a862f8c6cd45105be54286a549
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/webassembly/webassemblyrunconfiguration.cpp

@@ -50,8 +50,9 @@ static CommandLine emrunCommand(Target *target, const QString &browser, const QS
         // that the web server is killed when the application is stopped in Qt Creator.
         // On Non-windows, we prefer using the shell script, because that knows how to find the
         // right python (not part of emsdk). The shell script stays attached to the server process.
-        const FilePath interpreter = bc->environment().searchInPath(
-                    QLatin1String(HostOsInfo::isWindowsHost() ? "python" : "sh"));
+        const FilePath interpreter = HostOsInfo::isWindowsHost()
+                ? FilePath::fromUserInput(bc->environment().value("EMSDK_PYTHON"))
+                : bc->environment().searchInPath("sh");
         const QString emrunLaunchScript = HostOsInfo::isWindowsHost()
                 ? emrun.absolutePath() + "/" + emrun.baseName() + ".py"
                 : emrun.absoluteFilePath();