Allow the codeql-action to be run locally (#117)

* Allow the codeql-action to be run locally

This change allows the codeql-action to be run locally through
[act](https://github.com/nektos/act).

In order to run the action locally, you need to do two things:

1. Add the `CODEQL_LOCAL_RUN: true` environment variable. The only way
   I could figure out how to do this was to add it directly in the
   workflow file in an `env` block. It _should_ be possible to add it
   through a `.env` file and pass it to `act`, but I couldn't get it
   working.
2. Run this command `act -j codeql -s GITHUB_TOKEN=<MY_PAT>`

Setting the `CODEQL_LOCAL_RUN` env var will fill in missing env vars
that the action needs, but isn't set by `act`. It will also avoid
making api calls to github that would fail locally.

This is a refactoring discussed in
https://github.com/github/dsp-codeql/issues/36

CONTRIBUTING.md

@@ -26,6 +26,25 @@ This project also includes configuration to run tests from VSCode (with support
 
 To see the effect of your changes and to test them, push your changes in a branch and then look at the [Actions output](https://github.com/github/codeql-action/actions) for that branch.  You can also exercise the code locally by running the automated tests.
 
+### Running the action locally
+
+It is possible to run this action locally via [act](https://github.com/nektos/act) via the following steps:
+
+1. Create a GitHub [Personal Access Token](https://github.com/settings/tokens) (PAT).
+1. Install [act](https://github.com/nektos/act) v0.2.10 or greater.
+1. Add a `.env` file in the root of the project you are running:
+
+  ```bash
+  CODEQL_LOCAL_RUN=true
+
+  # Optional, for better logging
+  GITHUB_JOB=<ANY_JOB_NAME>
+  ```
+
+1. Run `act -j codeql -s GITHUB_TOKEN=<PAT>`
+
+Running locally will generate the CodeQL database and run all the queries, but it will avoid uploading and reporting results to GitHub. Note that this must be done on a repository that _consumes_ this action, not this repository. The use case is to debug failures of this action on specific repositories.
+
 ### Integration tests
 
 As well as the unit tests (see _Common tasks_ above), there are integration tests, defined in `.github/workflows/integration-testing.yml`.  These are run by a CI check.  Depending on the change you’re making, you may want to add a test to this file or extend an existing one.