[UI] fix artifact handler query parameter key

[Bobgy]

Error message when getting artifact:

Storage key is missing from artifact request

The bug was introduced in https://github.com/kubeflow/pipelines/pull/2745/files#diff-ac560cff381b58853d96359ba19782e9L179.
The query parameter should be "key", but it was renamed to "encodedKey".

/cc @eterna2

This PR depends on #2807.

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This change is 

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In response to this:

Error message when getting artifact:

Storage key is missing from artifact request

The bug was introduced in #2745

/cc @eterna2

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[Bobgy]

/test kubeflow-pipeline-e2e-test

[Bobgy]

/retest

[Bobgy]

/approve

[k8s-ci-robot]

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[Bobgy]

/test kubeflow-pipeline-e2e-test