Summary:
Update naming scheme for skaffold template files from .tmpl to .skfld.tmpl
in light of reuse of .tmpl extension

Test Plan:
pull,
     ```
    rm -rf skaffold_build/
    make skaffold-dev
     ```

Reviewers: michelle, zasgar, kgandhi

Reviewed By: michelle

Differential Revision: https://phab.corp.pixielabs.ai/D112

GitOrigin-RevId: 17e7e6dce44e78777bd97215ef56d0024fd1535f