If I have 5 points, can I win the game by holding to 7 points, and then activating Power Nexus to score another point by recycling 4 runes?

Yes, you can win the game in this scenario.

According to Rule 448.1.a.1, points earned from sources that are not "Conquer" or "Hold" are not subject to the special restrictions that apply to the final point.

Here is the breakdown of your turn:

1. Holding: You hold a battlefield to reach 7 points.
2. Power Nexus Ability: You activate the ability on Power Nexus to score 1 point by paying the cost. Because this point is earned through a card ability (not a "Conquer" or "Hold" action), it is not restricted by the "Final Point" rules (Rule 448.1.b).

Since you are moving from 7 points to 8 points using a non-restricted source, you successfully reach the victory score of 8 and win the game.