There are two dimensions to this problem and both matter:
- If this PhD student is passing a work in which you had a significant contribution (let alone if you are the main author) as their sole achievement, then it is plagiarism. However it is not plagiarism and it would be ethically acceptable if this PhD student is presenting the work as a joint work, since themselves also contributed to it. In this case they can also legitimately use the work in their PhD dissertation, provided that you are credited as co-author.
- In case it is actually plagiarism, if your advisor is knowingly covering or even encouraging it then they are at fault as well, and it's potentially even more serious for an experienced researcher: their duty includes ethical supervision of their students, and that would be a clear failure on their part.
- There is an important caveat that I think is worth mentioning: as a master student and then as a research assistant, there might be some level of ambiguity about who is the main author of the work. In particular in some fields a research assistant job might not be considered as original research work by itself, in the sense that the research assistant can be seen as providing some kind of technical expertise necessary for the research. This is a potential source of misunderstanding, especially if the technical expertise is outside the skills of the lead researcher. Depending on the specifics of the case, it might be important to consider the possibility that such a misunderstanding happened and clarify it with the supervisor.
Assuming that all the conditions for plagiarism are met, what you should do next is ultimately your own personal choice and you must be ready for the consequences of your decision. The academic system is not perfect, and sometimes a perfectly legitimate claim for credit is not the best course of action for your career. If you hope to continue in this research group and especially with this supervisor, it's clear that accusing them of plagiarism could seriously damage your chances. Even if you don't plan to continue in this particular place but somewhere else in academia, it's difficult to anticipate the consequences of an open conflict with a researcher in the field.
If you decide to move forward, the first step would be to try everything reasonable to convince the student and/or the supervisor to include your name in the work. It's usually much better for everyone if it can be solved internally. If they refuse, then you can seek the advice and possibly the mediation of somebody else: director of research, ombudsman or any other person in the institution with this role really.