[grafana] Don't fail assertNoLeakedSecrets check if a variable expansion operator is found

[bdalpe]

Checks for the presence of the following syntax anywhere in the setting (based on the documentation here: https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#variable-expansion):

${whatever} or $__<provider>{whatever}

If these are found, validation passes.

Fixes #2899

[bdalpe]

@Xtigyro or @zanhsieh, can you re-approve the workflows?

[stefanandres]

Thanks for the PR! Came here to tell this. All our grafana helm releases with okta are basically broken now because of

grafana/templates/deployment.yaml:36:28): Sensitive key 'auth.okta.client_secret' should not be defined explicitly in values. Use variable expansion instead

Even though we already use

client_secret: $__file{/etc/secrets/auth_okta/client_secret}

Reference to #2867 where this was implemented initially.

[bdalpe]

Will need someone with write access to merge as I don't have permissions.

[P-n-I]

lgtm, thanks!