feat: Add a skip_token_revoke input for configuring token revocation (#54)

Fixes https://github.com/actions/create-github-app-token/issues/55

Currently, `actions/create-github-app-token` always/unconditionally
revokes the installation access token in a `post` step, at the
completion of the current job. This prevents tokens from being used in
other jobs.

This PR makes this behavior configurable:
- When the `skip-token-revoke` input is not specified (i.e. by default),
the token is revoked in a `post` step (i.e. the current behavior).
- When the `skip-token-revoke` input is set to a truthy value (e.g.
`"true"`[^1]), the token is not revoked in a `post` step.

This PR adds a test for the `skip-token-revoke: "true"` case.

This is configurable in other app token actions, e.g.
[tibdex/github-app-token](3eb77c7243/README.md (L46-L47))
and
[wow-actions/use-app-token](cd772994fc/README.md (L132)).

[^1]: Note that `"false"` is also truthy: `Boolean("false")` is `true`.
If we think that’ll potentially confuse folks, I can require
`skip-token-revoke` to be set explicitly to `"true"`.

action.yml

@@ -17,6 +17,9 @@ inputs:
   repositories:
     description: "Repositories to install the GitHub App on (defaults to current repository if owner is unset)"
     required: false
+  skip_token_revoke:
+    description: "If truthy, the token will not be revoked when the current job is complete"
+    required: false
 outputs:
   token:
     description: "GitHub installation access token"