Resolving file read issue with binding-eval

[jmcshane]

Changes

This aligns the way that trigger eval parses the HTTP request file with the way that the
triggerrun binary operates. The file closing before reading the HTTP request body does
not work properly, so we have to do all of that within a single function

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Release Notes

NONE

Resolves #955

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
cmd/binding-eval/cmd/root.go	75.5%	75.0%	-0.5

[dibyom]

/approve
/lgtm

[tekton-robot]

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