I agree with mwestwood. By using the word "that", you would also add to the alliterative nature of your line--"That thriveth till thy". In addition, the "t" sound would again be emphasized in the world "light". Your line would be unified by one sound, thus emphasizing the idea that the earth will continue to "thrive" until it is no more. So, by being grammatically correct, you gain a line that is much more memorable. Good job.
Standard English today dictates that we use that to refer to people or things; which refers only things. So, "that" is acceptable.
Now, it appears that you are using sixteenth century English with the verbs that you have chosen. Nevertheless, "that" is still the relative pronoun to use.
Source: Prentice-Hall Grammar and Composition