Set interpreter only when necessary

[yuta-hayama]

If the value to be written with --set-intrepreter has already been set, the operation can be skipped. Similar checks are already performed by modifySoname() and can be implemented in setInterpreter().

This may avoid the problem that segfaults occur after multiple calls to patchelf --set-interpreter mentioned in the link below. https://github.com/NixOS/patchelf/issues/492#issuecomment-1610868690

It also prevents the addition of unnecessary LOAD segments each time patchelf --set-interpreter is called.