run-tests: use the same python version for shebang lines on Windows
The latest py3 is used if the minor number isn't specified. After running the
script to install all of the build dependencies, that moved the default from 3.8
to 3.9 on the CI system. That in turn caused a bunch of tests to be skipped
that were running prior, even when the test runner was invoked with py -3.8.
While we should almost always use the latest version, we really shouldn't make
it hard to test different versions or allow things to randomly break in subtle
ways like that.
Differential Revision: https://phab.mercurial-scm.org/D10702