Prevent Helm values schema validation to fetch external references when airgap

[LeoColomb]

What this PR does / why we need it:

By default, Helm will validate the values with a JSON schema if present in the chart.
Typically, if a subchart is defined as dependency, the top JSON schema includes references to subchart own JSON schema.
Often, this reference is a URI to download from “the outside”, which breaks Helm use when the environment is air-gapped.

Helm provides an option to disable values schema validation (SkipSchemaValidation), which can be use to prevent this to happening in this context.
Ideal would to check if the JSON schema (and its dependencies) contains external $refs but there is no easy way to achieve that.
In that extent, the proposed addition is fairly simple, even if it might disable JSON schema validation when it would have been possible without an error.

Which issue(s) this PR fixes:

No issue, but here is my investigation.

When running install with a Helm chart:

• client.Run() get called:
  

  kots/pkg/base/helm_v3.go

  Line 54 in 767739e

  rel, err := client.Run(chartRequested, renderOptions.HelmValues)

• Which is this function in Helm package: https://github.com/helm/helm/blob/11a9bf060bcf81475715fd721811f52b28c65872/pkg/action/install.go#L223
• Which expects a context, provided under the KOTS provided client object
• Without client.SkipSchemaValidation, client.Run() will start schema validation: https://github.com/helm/helm/blob/11a9bf060bcf81475715fd721811f52b28c65872/pkg/action/install.go#L304
• Which obviously render JSON schema: https://github.com/helm/helm/blob/b91a772d71340fcc250bd0616c8b6f596dca5746/pkg/chartutil/jsonschema.go#L76
• Which will download sub JSON schema via $ref: https://www.learnjsonschema.com/2020-12/core/ref/

Does this PR require a test?

I don't think so, but please let me know if it would be required.

Does this PR require a release note?

Prevent Helm to validate values schema on a air-gapped deployment 

Does this PR require documentation?

I don't think so.

---

References:

• feat(helm): add --skip-schema-validation flag to helm 'install', 'uprade' and 'lint' helm/helm#12743
• Unable to use the OSS chart in an airgapped environment netbox-community/netbox-chart#431
• @RangerRick

[CLAassistant]

All committers have signed the CLA.