go: update github.com/cyphar/filepath-securejoin to v0.3.4

[cyphar]

This includes a fix to avoid doing import "testing" in non-_test.go code.

Upstream changelog:

github.com/cyphar/filepath-securejoin v0.3.4

This release primarily includes a fix that blocked using
filepath-securejoin in Kubernetes.

- Previously, some testing mocks we had resulted in us doing import "testing"
  in non-_test.go code, which made some downstreams like Kubernetes unhappy.
  This has been fixed. (#32)

Thanks to all of the contributors who made this release possible:

 * Aleksa Sarai <[email protected]>
 * Stephen Kitt <[email protected]>

Signed-off-by: Aleksa Sarai <[email protected]>

Signed-off-by: Aleksa Sarai [email protected]