[semver:patch] Fix improperly dereferenced variable for GitHub

[robzr]

Checklist

• [x] All new jobs, commands, executors, parameters have descriptions
• [x] Examples have been added for any significant new features
• [x] README has been updated, if necessary

Motivation, issues

The variable handling for BOT_TOKEN was broken in the post-pr-comment command, when using this with dev-promote-from-commit-subject and parameter add-pr-comment: true, the step Post comment to GitHub pull request would show a failed curl api call:

{
  "message": "Not Found",
  "documentation_url": "https://docs.github.com/rest/reference/issues#create-an-issue-comment"
}

Traced this to the BOT_TOKEN environment variable not being dereferenced properly.

Description

Removed the extraneous $ prefix on the environment value; and added a bash dereference modifier (${!) to the script, tested (it works). Not sure what the point of the interim BOT_TOKEN environment variable was at all, instead of simply passing the << parameter.bot-token-variable >> right into the curl call, but I left it.

[gmemstr]

Interesting. I guess this is why that call never worked. Reviewing this now :)