pkg/instance: relax syzkaller path check
pkg/instance: relax syzkaller path check

Only check that syzkaller path is in GOPATH if we are going to build it.
syz-ci image testing does not have syzkaller path in GOPATH,
but it also does not build syzkaller.

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