[Airgapped env.] Add the airgapped tag into install script.

[LiuYuWei]

WHY:

1. Need to configure the PRIMEHUB_AIRGAPPED_IMAGE_PREFIX in env file. HOW:
2. Configure the variable when PRIMEHUB_AIRGAPPED is true.

PR checklist

• [ X ] Ensure you have added or ran the appropriate tests for your PR.
• [ X ] DCO signed

What type of PR is this? feature

What this PR does / why we need it: use Github Repo to install PrimeHub in air-gapped environment.

Which issue(s) this PR fixes:

Special notes for your reviewer: in env file, we will have the variable like this:

PRIMEHUB_AIRGAPPED=true
PRIMEHUB_AIRGAPPED_IMAGE_PREFIX=<airgapped-platform-url>

Does this PR introduce a user-facing change?:

- Add the air-gapped tag into install script.