Make the default github.com path consistent with other paths

[aeisenberg]

On util.ts:223, there is the following comment:

// Normalise path to having a trailing slash for consistency

However, I notice that the path defined by GITHUB_DOTCOM_URL
does not have a trailing slash. This PR fixes it.

I'm not 100% sure this is valid, so feel free to close if it doesn't make sense.

Though, if we don't go with this PR, then we should probably remove the requirement for a trailing / for other URIs.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.

[robertbrignull]

One thing to note is that constant currently has to match the GITHUB_SERVER_URL env var on actions on github.com. Look in actions-util.ts where we use this. But that check is a bit brittle and would probably be better off parsing the URL and checking the hostname instead of check for textual equality. If you change that usage then I think this change would be fine.

Something else I also spotted is in codeql.ts we do if (codeqlURL.startsWith(`${apiDetails.url}/`) which I think it going to be wrong if the API URL always has a trailing slash. Would be good to look into that.