Support enterprise hosted github instances

[collinmurd]

see #102

This adds support for GitHub Enterprise Server under a different hostname.

GITHUB_SERVER_URL is a default environment variable that is set on the runner to the url of whatever github instance it's associated with.

I only have my enterprise's environment to test with, this should be tested against github.com as well.

[monsagri]

Thank you for raising this @collinmurd!

I've left a couple of comments, one of them depending on us releasing an updated version this week

[collinmurd]

Thanks @monsagri! I'll keep an eye on your PR to upgrade go-github. After that's merged, I'll pull those changes here and test in my enterprise environment.

[monsagri]

Thanks @monsagri! I'll keep an eye on your PR to upgrade go-github. After that's merged, I'll pull those changes here and test in my enterprise environment.

Hey @collinmurd - We've just published the new version, would you be able to merge and give it a spin now?

[collinmurd]

Hey @monsagri. I'm running into an issue that may or may not be separate from this change.

The action is unable to create flag links in the app (with create-flag-links: true), and it fails with a seg fault:

Preprocessing diffs...
Scanning diff for references...
Summarizing results::debug::Setting outputs...
Processing comment...
Adding flag links...
  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x8c2b4c]
  
  goroutine 1 [running]:
  github.com/launchdarkly/find-code-references-in-pull-request/internal/ldclient.makeFlagLinkRep(0xc0001ec910, {0xc00045ade0, 0x12}, {0xc0009546d0, 0xa})
  	/app/internal/ldclient/flag_links.go:107 +0x1cc
  github.com/launchdarkly/find-code-references-in-pull-request/internal/ldclient.CreateFlagLinks(0xc0001b6360, {0xc00012a060?, 0xc00012a090?, 0xc00012a0c0?}, 0xc0001ec910)
  	/app/internal/ldclient/flag_links.go:[33](https://github.nwie.net/Nationwide/nwag-boot-utils/actions/runs/144478/job/2602746#step:5:37) +0x171
  main.main()
  	/app/main.go:105 +0xc15

I've confirmed this happens both before merging in the go-github update and after. Also confirmed my LD token has appropriate permissions. I missed this in my initial testing because it has an odd behavior.

The first time this runs on a PR, it successfully creates the LaunchDarkly flag references PR comment, but exits with the error above and fails to create flag links. If I re-run the workflow, the action will succeed without error, but still fail to create flag links. If re-run again, but delete the PR comment, it once again exits with error.

I'm doing some debugging, but this is my first time in Go, so it might be a slow process. If you have any ideas let me know!

At this point I'm not sure whether this issue is due to the integration with GitHub Enterprise, or if it's something to do with our self-hosted Actions runners, or even something to do with the change I made. So if you guys would prefer, I could close this PR for now and create an issue instead.