feat: run action as non-root user

[aslafy-z]

I'm running my own runners and get this error on checkout on subsequent builds:

=> Checkout
Run actions/checkout@v2
Syncing repository: user/repo
Getting Git version info
Deleting the contents of '/home/ec2-user/actions-runner/_work/user/repo'
##[error]Command failed: rm -rf "/home/ec2-user/actions-runner/_work/user/repo/helm"
rm: cannot remove '/home/ec2-user/actions-runner/_work/user/repo/charts/api/charts/postgresql-9.5.0.tgz': Permission denied

This fixes the issue.