The ornamental design for an outdoor petrun, as shown and described.
FIG. 1 is a perspective view of an outdoor pet run;
FIG. 2 is a leftside elevational view thereof;
FIG. 3 is a rightside elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a back view thereof; and,
FIG. 7 is a front view thereof.
The broken lines shown represent unclaimed subject matter and form no part of the claimed design and the areas between the broken lines do not form part of the claimed design.