buildstream/utils.py: Fix regex Deprecation Warning

Specify flags at the start of the expression as per the recommendation
of the standard library.

Without this patch, we currently get the following warning:

```
tests/examples/junctions.py::test_open_cross_junction_workspace
  /builds/BuildStream/buildstream/dist/buildstream/buildstream/utils.py:213: DeprecationWarning: Flags not at the start of the expression '\\/[^/]*\\Z(?ms)'
    regexer = re.compile(expression)
```