misc tests: Set specific versions in GH Action workflows (gem5#2335)

Setting these explicit will avoid our testing environments changing
without our knowledge thereby improving the stability of the workflows.

In the case of setting 'python-version' ( used in 'pre-commit'), doing
also avoids the following warning:

```
The `python-version` input is not set. The version of Python currently in `PATH` will be used.
```

Author: BobbyRBruce