bpo-30284: Fix regrtest for out of tree build

[vstinner]

Use a build/ directory in the build directory, not in the source
directory, since the source directory may be read-only and must not
be modified.

[vstinner]

I tested again with a fresh read-only source directory and I'm able to build Python and run tests with this change (whereas running tests fails without this change).

[vstinner]

Hum, it seems like the variable is not available on Windows:

C:\projects\cpython>"C:\projects\cpython\PCbuild\win32\python.exe"  -u -Wd -E -bb -m test  -uall -rwW --slowest --timeout 1200 -j0 
Traceback (most recent call last):
  ...
  File "C:\projects\cpython\lib\test\libregrtest\main.py", line 31, in <module>
    TEMPDIR = os.path.join(sysconfig.get_config_var('abs_builddir'), 'build')
  File "C:\projects\cpython\lib\ntpath.py", line 75, in join
    path = os.fspath(path)
TypeError: expected str, bytes or os.PathLike object, not NoneType

[vstinner]

I checked manually that it fixes the issue of read-only source tree: I'm now able to run tests from the build directory.