Deduplicate and sort termination message items

[imjasonh]

If a user or resource writes to /tekton/termination to append an output
result, later ones should win.

Also sort the deduplicated results for reproducibility.

cc @othomann

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[imjasonh]

/retest

[imjasonh]

/test pull-tekton-pipeline-build-tests

I0110 16:14:02.624] go get github.com/google/go-licenses: module github.com/google/go-licenses: Get https://proxy.golang.org/github.com/google/go-licenses/@v/list: net/http: TLS handshake timeout
I0110 16:14:02.670] /go/src/github.com/tektoncd/pipeline/vendor/github.com/tektoncd/plumbing/scripts/library.sh: line 325: go-licenses: command not found

[ghost]

/lgtm
/approve

[tekton-robot]

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This pull-request has been approved by: sbwsg

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[imjasonh]

I0110 19:44:30.497] go get github.com/google/go-licenses: module github.com/google/go-licenses: Get https://proxy.golang.org/github.com/google/go-licenses/@v/list: net/http: TLS handshake timeout
I0110 19:44:30.499] /go/src/github.com/tektoncd/pipeline/vendor/github.com/tektoncd/plumbing/scripts/library.sh: line 325: go-licenses: command not found

/test pull-tekton-pipeline-build-tests

[imjasonh]

/retest