go: build each package single-threaded (#53390)

I noticed that I was seeing the Go compiler build things in parallel even when I'd set `-j1 --cores 1`. It appears that the compiler, by default, uses the number of CPUs that are available to perform a build, while nixpkgs parallelizes at the directory level.

In order to change the fewest assumptions, this explicitly tells the Go compiler to run single-threaded. The flag's documentation is:

```
-p n
	the number of programs, such as build commands or
	test binaries, that can be run in parallel.
	The default is the number of CPUs available.
```

So this should function as expected. Feedback appreciated!