pkg/build: chmod output ssh key
pkg/build: chmod output ssh key

ssh is very picky about ssh key permissions.
Let's enforce the right persmissions without bothering user with this.

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