feat(env): improve GitHub Token handling (#77)

* fix: stop `env.GITHUB_TOKEN` from overriding `github_token` input

Previously, `GITHUB_API_TOKEN` and `GITHUB_TOKEN` were resolved as
`env.GITHUB_TOKEN || inputs.github_token`, meaning the environment
variable took priority over the action input. On Forgejo/Gitea
runners, `GITHUB_TOKEN` is automatically set to a non-GitHub token,
which silently overrode any user-provided github_token input and
caused GitHub API calls to fail.

* Improve GitHub API token input handling

Updated GitHub API token handling to use fallback values.

* Clarify 'github_token' description in README

Reworded the description of the 'github_token' parameter for clarity.

* Fix formatting of GitHub token description in README

* Update README with GitHub API token precedence

Added note on GitHub API token precedence in README.

* Update README.md

---------

Co-authored-by: Maxim Slipenko <maxim@slipenko.com>
Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>

action.yml

@@ -16,7 +16,6 @@ inputs:
   github_token:
     description: "GitHub API token"
     required: false
-    default: "${{ github.token }}"
 outputs:
   changelog:
     description: "output file"
@@ -37,14 +36,14 @@ runs:
         RUNNER_OS: ${{ runner.os }}
         RUNNER_ARCH: ${{ runner.arch }}
         VERSION: ${{ inputs.version }}
-        GITHUB_API_TOKEN: ${{ env.GITHUB_TOKEN || inputs.github_token }}
+        GITHUB_API_TOKEN: ${{ inputs.github_token || env.GITHUB_TOKEN || github.token }}
 
     - name: Run git-cliff
       id: run-git-cliff
       shell: bash
       run: ${GITHUB_ACTION_PATH}/run.sh --config=${{ inputs.config }} ${{ inputs.args }}
       env:
-        GITHUB_TOKEN: ${{ env.GITHUB_TOKEN || inputs.github_token }}
+        GITHUB_TOKEN: ${{ inputs.github_token || env.GITHUB_TOKEN || github.token }}
 
 branding:
   icon: "triangle"