Populate scripts.build

[harmony7]

For backwards compatibility Fastly CLI implies a scripts.build based on the language, but it's best to provide one in the fastly.toml.

[kpfleming]

While it's not related to this PR's changes, the cargo audit is failing due to a known vulnerability in the rsa crate. Since that vulnerability is not really applicable on Fastly Compute, and there is no fixed version available, the audit finding should be suppressed using the process documented here.

[harmony7]

@kpfleming thanks for going ahead and doing the patch!