🪧 Fix slack notification titles

[qgallouedec]

What does this PR do?

For some tests, we don't use matrix. So you can't set the name with, eg, Results with ${{ matrix.python-version }} on ${{ matrix.os }} with dev dependencies
It doesn't fail but renders like this:

Fixes # (issue)

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[edbeeching]

Thanks, do you mind giving a little more detail in the description about why this is needed?

[qgallouedec]

Thanks, do you mind giving a little more detail in the description about why this is needed?

Done, sorry about that