Get entryPoint for the invocation via the Github API

[ianlewis]

Depends on #47
Fixes #11

Before the change the predicate.invocation.configSource.entryPoint can have name of the user workflow (not the reusable workflow) specified in the workflow yaml:

"predicate": {
    "invocation": {
        "entryPoint": "SLSA provenance"
        ...
      },
...

After this change it's a path to the workflow:

"predicate": {
    "invocation": {
      "configSource": {
        "entryPoint": ".github/workflows/provenance-only.yml"
        ...
      },
...

[laurentsimon]

Thanks

[joshuagl]

This is great, thanks. Thanks also for showing before and after fragments of the provenance.