pkg/build: use fixed key when signing modules
pkg/build: use fixed key when signing modules

Generating new key for every build causes problems to having
reproducible builds. To avoid this use fixed key.

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