lux: fix test

[stefanb]

Old test image URL https://github.githubassets.com/images/modules/site/icons/footer/github-logo.svg gives 404 now due to github site changes:

Testing lux
  ==> /opt/homebrew/Cellar/lux/0.22.0/bin/lux -i https://github.githubassets.com/images/modules/site/icons/footer/github-logo.svg
  Downloading https://github.githubassets.com/images/modules/site/icons/footer/github-logo.svg error:
  https://github.githubassets.com/images/modules/site/icons/footer/github-logo.svg request error: HTTP 404

Observed in

• go 1.21.7 #161976

but will show up also in

• go 1.22.0 #157782

in the next test run

Changed to an image https://upload.wikimedia.org/wikipedia/commons/c/c2/GitHub_Invertocat_Logo.svg that is not subject to an unexpected site redesign.

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