provider/tekton: fix service account name location
Tekton v1beta1 PipelineRun objects[1] had the service account specified at
the top level of spec, eg:
```yaml
spec:
serviceAccountName: tack
```
However Tekton v1 PipelineRun objects have the service account inside a
`taskRunTemplate` object:
```yaml
spec:
taskRunTemplate:
serviceAccountName: tack
```
This change properly places the service account name in the right place
for Tekton v1 PipelineRun objects so that Tack can properly annotate CI
pipelines with the right service account.
[1]: https://tekton.dev/docs/pipelines/pipelineruns/#mapping-serviceaccount-credentials-to-tasks
Signed-off-by: Xe Iaso <me@xeiaso.net>