pkg/build: update fuchsia fx arguments
pkg/build: update fuchsia fx arguments

We've once again changed the way you define a build via fx; update it in
Syzkaller too.
google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller