How to keep a randomization formula from randomizing at each change in the sheet?

I have this cute formula:

=sort(A1:A416,arrayFormula(randbetween(sign(row(A1:A416)),C1)),true)

I thought I could change the randomization event when I changed the value in C1, but now I see this is changing continuously when I update the sheet with other values.

How can I keep this from happening?

I only want the randomization to happen when I change the seed in C1.