Actually, depending on your source, some grammar guides use infinitive phrase and infinitive clause interchangably. Let us remind ourselves of what an infinitive phrase is. An infinitive phrase beings with an infinitive, which is a verb starting with the word "to" and followed by the simple form of that verb. It will also include objects/modifiers and can act as a noun. In the example you have given, the infinitive, "to find a good summer job," acts as a noun because it is the subject of the sentence, as it is the "goal" of the speaker. Therefore this quote is an infinitive phrase because it includes an infinitive verb and the phrase as a whole acts as a noun rather than the verb.