x/build: adjust LUCI load at openbsd-ppc64 builder

[n2vigo]

As agreed with @dmitshur and Michael K, add GOMAXPROCS to try reducing the load average on the builder machine.

[gopherbot]

Change https://go.dev/cl/625317 mentions this issue: x/build: adjust LUCI load at openbsd-ppc64 builder

[gopherbot]

Change https://go.dev/cl/633996 mentions this issue: main.star: adjust LUCI load at openbsd-ppc64 builder