Add --insecure flag to GraphQL schema validation

[tjwiebell]

Description

Add --insecure flag to download-schema script.

Related Issue

Closes #1311

Verification Steps

1. Setup Magento backend with self-signed or insecure cert
2. yarn build
3. Build successful

How have YOU tested this?

Ran the verification steps

Proposed Labels for Change Type/Package

• major (e.g x.0.0 - a breaking change)
• minor (e.g 0.x.0 - a backwards compatible addition)
• patch (e.g 0.0.x - a bug fix)

Checklist:

• I have updated the documentation accordingly, if necessary.
• I have added tests to cover my changes, if necessary.

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[jimbo]

Makes sense to me.

[PWAStudioBot]

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