[test][Dexter] Fix test failure if space in python path

The '%dexter_regression_test' substitution was missing quotes around the
python executable, unlike other substitutions of a similar nature in the
file. This changes fixes the issue.

Differential Revision: https://reviews.llvm.org/D96420

Reviewed by: jmorse, aganea

GitOrigin-RevId: a31eae840525e9292a3a42c1fdac3fc594f42949
1 file changed