Support customized issuer values for GitHub Enterprise Cloud

[isometry]

The action is currently incompatible with any organisation belonging to a GitHub Enterprise Cloud instance that has followed the hardening guidance to customize the issuer value:

Error: Failed to get ID token: unexpected "iss" claim value

With the GitHub Actions OIDC Custom Issuer Policy enabled (i.e. include_enterprise_slug=true), the Enterprise Cloud issuer becomes https://token.actions.githubusercontent.com/${enterpriseSlug}.

Ideally, the action would dynamically retrieve the correct enterpriseSlug given the runtime context; alternatively it should support an optional enterpriseSlug input parameter to be suffixed to the calculated issuer.

[bdehamer]

Definitely something we need to fix.

I don't think there is a good way to determine whether the issuer customization is enabled for a given account, so will probably end-up with an include_enterprise_slug input param.

[bdehamer]

This should be resolved with the v1.4.3 release. We didn't end-up adding a new input parameter -- it should just work whether or not you have customized the issuer URL for your organization.