Provide the ability to configure GitHub App PEM contents via ENV VAR

[scottford-io]

Is your feature request related to a problem? Please describe.
I want to be able to configure Google Cloud Build to scan my GitHub organization using the Mondoo GitHub App. To do this, I need to store the PEM file created by the GitHub App in Google Cloud Secrets Manager and then use that file to authenticate my scan.

Today, cnspec provides the --app-private-key flag which expects a path to the PEM file. To use that with Google Cloud Build I need to retrieve the PEM file from Secrets Manager, cat the contents to a file, and then pass that file path to the --app-private-key flag.

Describe the solution you'd like
I would like to be able to pull the PEM file from Secrets Manager and set its contents directly via an ENV VAR.