pkg/build: Add ssh keys for fuchsia
pkg/build: Add ssh keys for fuchsia

This change modifies the way fuchsia is built to add the ssh keys to the
zbi image after building it. Previously that was done via the
"extra_ssh_keys" argument to the build system, but that was removed
recently.

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