ci: Fix Python 2 in CI stalling tests causing flaky failures

In MacVim CI, we install Python 2 for backwards compatibility reasons,
but CI tests prefer Python 2 so we end up launching that instead, and it
seems like that was the cause for some flaky failures (e.g.
`Test_terminal_aucmd_on_close`). This is probably due to the fact that
Python 2 is only availble in x86 and launching it on an ARM Mac needs to
go through Rosetta which takes time and fails the timeout.

src/testdir/shared.vim

@@ -25,10 +25,10 @@ func PythonProg()
     if !(has('job') || executable('pkill'))
       return ''
     endif
-    if executable('python')
-      let s:python = 'python'
-    elseif executable('python3')
+    if executable('python3')
       let s:python = 'python3'
+    elseif executable('python')
+      let s:python = 'python'
     else
       return ''
     end