Make PEP 517 mutation tests use pinned old setuptools (#80262)

* Pin setuptools to lowest supported @ PEP 517 test

This allows catching the behavior of builds under old setuptools.

* Stop invoking `setup.py install` in tests

This is not the part we care about since it involves dealing with the
external runtime dependencies rather than building our source
distribution.

test/integration/targets/canonical-pep517-self-packaging/runme_test.py

@@ -207,18 +207,23 @@ def test_installing_sdist_build_with_modern_deps_to_old_env(
         with TarFile.gzopen(tmp_path_sdist_w_modern_tools) as sdist_fd:
             sdist_fd.extractall(path=tmp_dir_unpacked_sdist_root)
 
+        pip_install(
+            venv_python_exe, 'setuptools',
+            env_vars={
+                'PIP_CONSTRAINT': str(LOWEST_SUPPORTED_BUILD_DEPS_FILE),
+            },
+        )
         with _chdir_cm(tmp_dir_unpacked_sdist_path):
             run_with_venv_python(
                 venv_python_exe, 'setup.py', 'sdist',
                 env_vars={'PATH': environ['PATH']},
             )
-            run_with_venv_python(
-                venv_python_exe, 'setup.py', 'install',
-                env_vars={'PATH': environ['PATH']},
-            )
     else:
         pip_install(
             venv_python_exe, str(tmp_path_sdist_w_modern_tools), '--no-deps',
+            env_vars={
+                'PIP_CONSTRAINT': str(LOWEST_SUPPORTED_BUILD_DEPS_FILE),
+            },
         )
 
     # Smoke test — installing an sdist with pip that does not support invoking