Set SPECTRAL_DSN environment from JS instead of end-user workflow file

[chkp-jrosenboom]

The action originally suggests that the user should first set SPECTRAL_DSN from GitHub Secrets, then pass this environment variable as an input to the action.

...
env:
  SPECTRAL_DSN: ${{ secrets.SPECTRAL_DSN }}
...
        uses: spectralops/spectral-github-action@v3
        with:
          spectral-dsn: ${{ env.SPECTRAL_DSN }}

If the user skips configuring the environment variable in their workflow file, and passes the input directly to the action, the installation will succeed because it uses the value read from inputs.spectral-dsn here.

The execution of the scan itself does not use the same value, and depends on SPECTRAL_DSN being set as an environment variable. Because of this, the scan fails in this scenario:

🙅‍ error: You need to provide your key (Spectral DSN).

This PR

• changes the action to set SPECTRAL_DSN from the input variable in index.js
• updates examples in README.md
• updates example action

It removes the need for the user to provide SPECTRAL_DSN both as an input to the action and as an environment variable.

[chkp-jrosenboom]

This PR requires a new tag 'v4' to be created after merging.
The examples will point to that tag already.