ci: write only failed test output to GitHub Actions

[abhinav]

gotestsum's github-actions formatter writes grouped output
for all tests across all packages to the logs.
This ends up really slowing down GitHub Actions log browser
because of the sheer volume of output.

Switch to the github-actions-fails formatter
implemented in gotestyourself/gotestsum#568
to fix this.

[skip changelog]: no user facing changes

[abhinav]

This change is part of the following stack:

• ci: write only failed test output to GitHub Actions #1194 ◀

_{Change managed by git-spice.}