Replace non-functional Boost download URLs

[speth]

Changes proposed in this pull request

Boost downloads from jfrog.io have stopped working. This seems to be approximately an annual occurrence. Switching to downloads from GitHub releases should hopefully be a more stable source.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

I know it’s still in draft, but this looks all good. Had looked at the same repo when failures first started.