Request: Adding Github Host to context to enable writing actions for github enterprise

[adam-bratin]

Describe the enhancement
Currently most actions are written to only work with repos hosted on Github.com. As Github enterprise 2.22.0 has been released with github actions beta I was hoping to enable adding to the github context a host variable. This will enable actions using context from '@actions/github' to set up a gh client consistently without the action developer needing to know where the action is being used.
Code Snippet
I am assuming adding a line after here

toolkit/packages/github/src/context.ts

Line 45 in 71b19c1

this.runId = parseInt(process.env.GITHUB_RUN_ID as string, 10)

:

this.host = process.env.GITHUB_HOST ?? "https://api.github.com" as string 

Additional information