fix(ci): pin ko binary to v0.18.1 for release-v0.20.x e2e tests

[infernus01]

Changes

Pin ko binary version to v0.18.1 in the e2e workflow for
release-v0.20.x.

Problem

The setup-ko action had no version: input, so it defaulted to
latest-release. ko v0.19.1 introduced a security change that
rejects kodata symlinks resolving outside the kodata root:

kodata symlink "HEAD" resolves to ".git/HEAD" which is outside
the kodata root

This breaks ko apply because cmd/controller/kodata/HEAD and
cmd/controller/kodata/refs are symlinks into .git/ for version
embedding. All e2e tests fail since the controller image can't be
built.

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Release Notes

NONE

[anithapriyanatarajan]

/lgtm

[anithapriyanatarajan]

@jkhelil - I think pinning is ok on old branch

[jkhelil]

probably yes

[jkhelil]

/approve

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