What is golden ratio doing in this computer code?

If you look at the code you find that the routine is from Numerical recipes. And if you look there you find the comment:According to Knuth, any large MBIG, and any smaller (but still large) MSEED can be substituted for the above values.

In fact the NR routine is derived from Knuth's subtractive generator IN55 (described in Seminumerical Algorithms 3.6), which also explains the magic 55 in the NR and MS code.

It is rather common, when one needs a single "random" large constant that doesn't need to have any special properties (aside possibly from being too simple), to use digits from popular numbers. $\pi$ is the most common, I think, but the golden ratio rates as being a popular number so I would be unsurprised to see it used too.