Devops: fix action for ubuntu-latest and invariant culture in tests

[tesar-tech]

Original problem: #5940 (comment)

The error was this:

Assert.Equal() Failure: Strings differ
                              ↓ (pos 19)
Expected: "12/31/9999 11:59:59 PM"
Actual:   "12/31/9999 11:59:59 PM"
                              ↑ (pos 19)

Explanation:

At position 19, the space before "PM" is different:

    The expected value has a regular space (U+0020).
    The actual value has a narrow no-break space (U+202F), which looks visually similar but is a different Unicode character.

It is culture dependent. I don't exactly know why that was happening in ubuntu 24 not 22...

This fix makes the tests more deterministic.

I re-discovered the issue by an accident. And fixing it till it's in fresh memory...