installation.md: fix broken ubuntu link

[cakebaker]

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[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.