The example in previous versions of the Go math/rand package suggested using time.Now().UnixNano() to seed the RNG source since that's essentially a random int64.
*BSD, newer macOS, and GNU libc: int getentropy(void *buffer, size_t length);
Linux 3.19+: ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);
iOS and macOS 10.7+: int SecRandomCopyBytes(SecRandomRef, size_t, uint8_t *);
Fuchsia: void zx_cprng_draw(void* buffer, size_t buffer_size);
Any *nix: Read from "/dev/random" (or "/dev/urandom" under the assumption that your system has already enough entropy).
Windows: There are different functions/libraries depending on the Windows version and some of them are a complicated multi-step mess.
Some time ago I wrote a C library that abstracts that away just for fun: https://github.com/panzi/portable_get_random*
https://en.m.wikipedia.org/wiki/Hardware_random_number_gener...
Maybe we're in the first batch of simulations, and the tester came along and asks why they're all identical. The cosmic coder then realises that they forgot to call the function to seed the prng.