Substitute deprecated rand.Seed() in Go 1.20

[henry118]

rand.Seed is deprecated since Go 1.20.

Compiling this package with Go 1.20 resulted in the following linter error:

manifest_test.go:44:2: SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative has been available since Go 1.0: Programs that call Seed and then expect a specific sequence of results from the global random source (using functions such as Int) can be broken when a dependency changes how much it consumes from the global random source. To avoid such breakages, programs that need a specific result sequence should use NewRand(NewSource(seed)) to obtain a random generator that other packages cannot access. (staticcheck)
rand.Seed(1)
^
make: *** [Makefile:48: lint] Error 1