Cherry Pick into 0.13.x: Externalize Affinity Assistant image

[bobcatfish]

Changes

Cherry picking this into the 0.13.x release so that anyone downstream of tekton building on it can control the image being used and is able to use the 0.13.x releases

Container images should not be hardcoded.
This commit externalize what image should be used for the Affinity Assistant.

/kind misc

(cherry picked from commit 3c1c49d)

[tekton-robot]

This PR cannot be merged: expecting exactly one kind/ label

Available kind/ labels are:

kind/bug: Categorizes issue or PR as related to a bug.
kind/flake: Categorizes issue or PR as related to a flakey test
kind/cleanup: Categorizes issue or PR as related to cleaning up code, process, or technical debt.
kind/design: Categorizes issue or PR as related to design.
kind/documentation: Categorizes issue or PR as related to documentation.
kind/feature: Categorizes issue or PR as related to a new feature.
kind/misc: Categorizes issue or PR as a miscellaneuous one.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/pipelinerun/affinity_assistant.go	91.7%	92.0%	0.3
pkg/reconciler/pipelinerun/pipelinerun.go	81.7%	80.5%	-1.2
pkg/reconciler/taskrun/taskrun.go	77.0%	76.0%	-1.0

[vdemeester]

/meow
/cc @afrittoli @sbwsg

[tekton-robot]

@vdemeester:

In response to this:

/meow
/cc @afrittoli @sbwsg

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[tekton-robot]

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[ghost]

/lgtm