Fix release task with pipeline 0.7 syntax
Fix release task with pipeline 0.7 syntax

Pipeline 0.7 changes the variable interpolation
syntax from `${}` to `$()`. This commit updtes
the `publish-tekton-triggers` task with the new
syntax.

Signed-off-by: Dibyo Mukherjee <7d445be0c0af41c00c371a1f7b3669c3331901f5@google.com>

tektoncd/triggers,tektoncd/triggers