Clarify that we cannot test Checks API locally

Author: mumoshu

.gitignore

@@ -2,3 +2,4 @@
 .idea
 bin/
 *.json
+.envrc

cmd/checks/README.md

@@ -25,6 +25,13 @@ Usage of bin/checks:
 
 ## Running locally
 
+> This is just here to show the ideal developer experience. The reality is that you can't test `checks` locally.
+>
+> That's due to limitations in GitHub.
+> As Checks API requires a GitHub App installation token(not bearer token you're probably familiar with) to access
+> and there's currently no way to obtain an installation token locally,
+> you can't test this locally.
+
 Capture actual webhook payloads for `pull_request`, `check_suite`, `check_run` events by running `cat /github/workflow/event.json` on GitHub Actions.
 
 Install this workflow onto your test repository:
@@ -60,11 +67,11 @@ Then build and run `checks` against json files containing captured events:
 $ make build/checks
 
 # Typically this is run on Actions with `on: pull_requqest`
-$ GITHUB_EVENT_PATH=$(pwd)/pull_request_event.json bin/checks
+$ GITHUB_TOKEN_TYPE=bearer GITHUB_EVENT_PATH=$(pwd)/pull_request_event.json GITHUB_EVENT_NAME=pull_request bin/checks
 
 # `on: check_suite`
-$ GITHUB_EVENT_PATH=$(pwd)/check_suite_event.json bin/checks -create-run foo -create-run bar
+$ GITHUB_TOKEN_TYPE=bearer GITHUB_EVENT_PATH=$(pwd)/check_suite_event.json bin/checks -create-run foo -create-run bar
 
 # `on: check_run`
-$ GITHUB_EVENT_PATH=$(pwd)/check_run_event.json bin/checks -run foo -- actions pullvet ...
+$ GITHUB_TOKEN_TYPE=bearer GITHUB_EVENT_PATH=$(pwd)/check_run_event.json bin/checks -run foo -- actions pullvet ...
 ```

pkg/checks/checks.go

@@ -233,7 +233,11 @@ func (c *Command) instTokenClient() (*github.Client, error) {
 // instTokenClient uses an installation token to authenticate to the Github API.
 func instTokenClient(instToken, baseURL, uploadURL string) (*github.Client, error) {
 	// For installation tokens, Github uses a different token type ("token" instead of "bearer")
-	t := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: instToken, TokenType: "token"})
+	tokenType := "token"
+	if os.Getenv("GITHUB_TOKEN_TYPE") != "" {
+		tokenType = os.Getenv("GITHUB_TOKEN_TYPE")
+	}
+	t := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: instToken, TokenType: tokenType})
 	c := context.Background()
 	tc := oauth2.NewClient(c, t)
 	if baseURL != "" {