bpo-40432 Fix MSBuild project for Pegen grammars

[tonybaloney]

The additional tasks in the MSBuild project for pegen regeneration are not functional:

• Setting PYTHONPATH= inline cannot be done in Windows using that method
• The task does not inherit environment variables that way
• The path to the peg_generator module is in Unix path format

Cc @pablogsal

https://bugs.python.org/issue40432

[tonybaloney]

before

c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(175,5): error MSB3073: The command ""c:\Users\anthonysh
aw\source\repos\cpython\PCbuild\win32\python.exe" -m pegen -q c "c:\Users\anthonyshaw\source\repos\cpython\Grammar\pyth
on.gram" "c:\Users\anthonyshaw\source\repos\cpython\Grammar\Tokens" -o "c:\Users\anthonyshaw\source\repos\cpython\PCbui
ld\obj\39win32_Release\regen\parse.c"" exited with code 1.

After

Build succeeded.

c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(167,5): warning : Grammar updated. You will need to reb
uild pythoncore to see the changes.
c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(184,5): warning : Python-ast.h updated. You will need t
o rebuild pythoncore to see the changes.
c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(192,5): warning : Python-ast.c updated. You will need t
o rebuild pythoncore to see the changes.
c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(200,5): warning : Opcodes updated. You will need to reb
uild pythoncore to see the changes.
c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(218,5): warning : Keywords updated. You will need to re
build pythoncore to see the changes.
c:\Users\anthonyshaw\source\repos\cpython\PCbuild\regen.vcxproj(176,5): warning : Pegen updated. You will need to rebui
ld pythoncore to see the changes.
    6 Warning(s)
    0 Error(s)

[pablogsal]

Looks great!

Thanks for checking and the quick fix @tonybaloney ! ❤️