Pass --isolated to 'pip install'

Fixes pypa#90.

[The `--isolated` option](https://github.com/pypa/pip/blob/89a51a6fefec826256fb334ea6244dfb0b3455a0/src/pip/_internal/cli/cmdoptions.py#L153) ignores environment variables and user configuration, so only the passed command will be run. This fixes `python -m build` for users with `PIP_REQUIRE_VIRTUALENV`, and maybe some other cases.

I'm not sure if it will break other flows though, such as if a proxy is configured.

src/build/env.py

@@ -170,8 +170,8 @@ def install(self, requirements):  # type: (Iterable[str]) -> None
             req_file.write(os.linesep.join(requirements))
             req_file.close()
             cmd = [
-                sys.executable, '-m', 'pip', 'install', '--prefix',
-                self.path, '-r', os.path.abspath(req_file.name)
+                sys.executable, '-m', 'pip', 'install', '--isolated',
+                '--prefix', self.path, '-r', os.path.abspath(req_file.name)
             ]
             subprocess.check_call(cmd)
             os.unlink(req_file.name)

tests/test_env.py

@@ -64,8 +64,9 @@ def test_isolated_environment_install(mocker):
         if sys.version_info[:2] != (3, 5):
             subprocess.check_call.assert_called()
         args = subprocess.check_call.call_args[0][0]
-        assert args[:7] == [
-            sys.executable, '-m', 'pip', 'install', '--prefix', env.path, '-r'
+        assert args[:8] == [
+            sys.executable, '-m', 'pip', 'install', '--isolated',
+            '--prefix', env.path, '-r'
         ]