Need to pull Traefik helm chart from new charts URL

[brandond]

Environmental Info:
K3s Version:
N/A

Node(s) CPU architecture, OS, and Version:
N/A

Cluster Configuration:
N/A

Describe the bug:
The K3s build scripts still pull the Traefik chart from the deprecated Helm Charts stable repo.
https://github.com/k3s-io/k3s/blob/v1.20.0-rc5+k3s1/scripts/download#L16

Steps To Reproduce:

• Build K3s
• Note that the traefik chart tarball fails to download.

Expected behavior:
Helm chart is downloaded successfully and build succeeds

Actual behavior:
Helm chart is downloaded from the old repo, which is offline as of 2020-12-21

Additional context / logs:
https://drone-publish.k3s.io/k3s-io/k3s/87/1/2

[rancher-max]

Validated traefik is pulled correctly in v1.20.0+k3s2.
Need to validate in all backports still before closing.

[rancher-max]

Validated all backports - traefik pulled correctly and install succeeds.

Created #2777 to address pulling from the new stable chart in helm install pod logs.