Use None as Environment object build_dir in detect_system_compiler()

The Environment object constructor accepts None as build_dir (for quite
a while now), so don't bother with creating a temporary directory for
use as the build_dir, if we're not going to need it.

Future work: Environment.__init__() sets scratch_dir to '' if build_dir
is None, which seems a little wonky, as it isn't a path.

run_project_tests.py

@@ -1386,14 +1386,13 @@ def check_meson_commands_work(use_tmpdir: bool, extra_args: T.List[str]) -> None
 def detect_system_compiler(options: 'CompilerArgumentType') -> None:
     global host_c_compiler, compiler_id_map
 
-    with TemporaryDirectoryWinProof(prefix='b ', dir=None if options.use_tmpdir else '.') as build_dir:
     fake_opts = get_fake_options('/')
     if options.cross_file:
         fake_opts.cross_file = [options.cross_file]
     if options.native_file:
         fake_opts.native_file = [options.native_file]
 
-        env = environment.Environment(None, build_dir, fake_opts)
+    env = environment.Environment(None, None, fake_opts)
 
     print_compilers(env, MachineChoice.HOST)
     if options.cross_file: