docs/fuchsia: Build syz-executor inside fuchsia.
docs/fuchsia: Build syz-executor inside fuchsia.

This commit documents how to build syz-executor as part of fuchsia.
Basically you just need to add two flags to the fx set command: one for
adding syz-executor, and one for telling fuchsia to use your local
syzkaller repository instead of the one in third_party.

xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller