env: prevent warning from being emitted on venv creation

After python/cpython@6811fda
the venv module produces spurious warnings for venv paths which contain
DOS-encoded parts e.g. "USER~1" in "C:\Users\USER~1".
`tempfile.gettempdir()` returns legacy paths like these for
user temp dirs.

MRE:

    python -c "import tempfile
    import venv

    venv.create(tempfile.mkdtemp())"

    Actual environment location may have moved due to redirects, links or junctions.
    Requested location: "C:\Users\RUNNER~1\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"
    Actual location:    "C:\Users\runneradmin\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"

Closes pypa#413.

src/build/env.py

@@ -92,7 +92,9 @@ def __enter__(self) -> IsolatedEnv:
 
         :return: The isolated build environment
         """
-        self._path = tempfile.mkdtemp(prefix='build-env-')
+        # ``realpath`` is used to prevent warning from being emitted that
+        # the venv location has moved on Windows.
+        self._path = os.path.realpath(tempfile.mkdtemp(prefix='build-env-'))
         try:
             # use virtualenv when available (as it's faster than venv)
             if _should_use_virtualenv():