Show message "Press ctrl c to exit" on forward manager start

[gsquared94]

Fixes: #3415

Description
Show message "Press ctrl c to exit" on forward manager start

User facing changes (remove if N/A)

[codecov]

Codecov Report

Merging #4113 into master will increase coverage by 0.31%.
The diff coverage is 0.00%.

Impacted Files	Coverage Δ
pkg/skaffold/runner/build_deploy.go	56.12% <0.00%> (-1.49%)	⬇️
pkg/skaffold/deploy/labeller.go	50.00% <0.00%> (-50.00%)	⬇️
pkg/skaffold/errors/errors.go	76.19% <0.00%> (-23.81%)	⬇️
cmd/skaffold/app/cmd/deploy.go	69.44% <0.00%> (-11.21%)	⬇️
...skaffold/kubernetes/debugging/container_manager.go	43.28% <0.00%> (-5.05%)	⬇️
cmd/skaffold/app/cmd/commands.go	95.12% <0.00%> (-4.88%)	⬇️
pkg/skaffold/deploy/kubectl/cli.go	90.90% <0.00%> (-3.83%)	⬇️
pkg/skaffold/runner/debugging.go	30.00% <0.00%> (-3.34%)	⬇️
pkg/skaffold/deploy/status_check.go	65.06% <0.00%> (-3.25%)	⬇️
pkg/skaffold/runner/dev.go	65.11% <0.00%> (-0.38%)	⬇️
... and 25 more

[briandealwis]

Thanks for your contribution! Just one little nit.