remove new compiler warnings

[octaeder]

C:/msys64/home/WDAG/texstudio-org/texstudio/src/buildmanager.cpp:1745:19: warning: ignoring return value of 'bool QTemporaryFile::open()', declared with attribute 'nodiscard' [-Wunused-result]
 1745 |         temp->open();
      |         ~~~~~~~~~~^~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/buildmanager.cpp:1851:41: warning: ignoring return value of 'bool QTemporaryFile::open()', declared with attribute 'nodiscard' [-Wunused-result]
 1851 |                                 tf->open();
      |                                 ~~~~~~~~^~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/buildmanager.cpp:1907:17: warning: ignoring return value of 'bool QTemporaryFile::open()', declared with attribute 'nodiscard' [-Wunused-result]
 1907 |         tf->open();
      |         ~~~~~~~~^~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/configmanager.cpp:2910:18: warning: ignoring return value of 'virtual bool QFile::open(QIODeviceBase::OpenMode)', declared with attribute 'nodiscard' [-Wunused-result]
 2910 |         file.open(QFile::ReadOnly);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/latexstyleparser.cpp:824:17: warning: ignoring return value of 'bool QTemporaryFile::open()', declared with attribute 'nodiscard' [-Wunused-result]
  824 |         tf->open();
      |         ~~~~~~~~^~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/pdfsplittool.cpp:330:19: warning: ignoring return value of 'bool QTemporaryFile::open()', declared with attribute 'nodiscard' [-Wunused-result]
  330 |         temp->open();
      |         ~~~~~~~~~~^~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/searchquery.cpp:116:11: warning: ignoring return value of 'virtual bool QFile::open(QIODeviceBase::OpenMode)', declared with attribute 'nodiscard' [-Wunused-result]
  116 |     f.open(QIODevice::ReadOnly);
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~~~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/texstudio.cpp:7744:21: warning: ignoring return value of 'virtual bool QFile::open(QIODeviceBase::OpenMode)', declared with attribute 'nodiscard' [-Wunused-result]
 7744 |         xmlFile.open(QIODevice::ReadOnly);
      |         ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/texstudio.cpp:7774:22: warning: ignoring return value of 'virtual bool QFile::open(QIODeviceBase::OpenMode)', declared with attribute 'nodiscard' [-Wunused-result]
 7774 |         xmlFile2.open(QIODevice::ReadOnly);
      |         ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
C:/msys64/home/WDAG/texstudio-org/texstudio/src/texstudio.cpp:7788:26: warning: ignoring return value of 'virtual bool QFile::open(QIODeviceBase::OpenMode)', declared with attribute 'nodiscard' [-Wunused-result]
 7788 |             xmlFile3.open(QIODevice::ReadOnly);
      |             ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

[sunderme]

If the warnings are addressed, I would prefer to do it right.
The nodiscard flag means that (theoretically) it is possible that file creation fails and the program should act accordingly.
so, here the return value needs to be checked.