gh-37065: Installation guide: Use .. envvar:: to define environment…

… variables

    
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This turns references ``:envvar:`....` `` into hyperlinks to the place
of the definition. https://deploy-preview-37065--
sagemath.netlify.app/html/en/installation/source#environment-variables

Also improving the structure of the section, and some smaller updates.

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URL: #37065
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee