Add install_sct.bat as a release asset during create-release.yml workflow

[joshuacwnewton]

Checklist

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Description

This PR updates the create release workflow so that it will modify install_sct.bat to make it release-ready, then attach it as an asset to the release.

The changes in this PR were tested on my fork to create https://github.com/joshuacwnewton/spinalcordtoolbox/releases/tag/5.7.5, which indeed has the batch script attached as a release asset, with the correct line changed:

  rem Set git ref. If no git ref is specified when calling `install_sct.bat`, use a default instead.
  if [%1]==[] (
    set git_ref=5.7.5
  ) else (
    set git_ref=%1
  )

Linked issues

Fixes #3763.

[mguaypaq]

Overall this looks fine, so I'm approving this. My questions are really more out of curiosity.

I'm also noticing that ncipollo/release-action@v1 has another output parameter called upload_url (see here), which the GitHub docs say we could directly use with curl to upload assets (example in the sidebar here), but that's probably more trouble to debug than just using an existing action.