Don't erroneously emit warning about running pip as root

Check the uid using getuid() on platforms other than Linux and Mac OS before
warning about running pip as root.

Before this change, pip was emitting this warning even when it was run as a
non-root user.

news/10565.bugfix.rst

@@ -0,0 +1 @@
+Tweak running-as-root detection, to check ``os.getuid`` if it exists, on Unix-y and non-Linux/non-MacOS machines.

src/pip/_internal/cli/req_command.py

@@ -173,9 +173,10 @@ def warn_if_run_as_root() -> None:
     # checks: https://mypy.readthedocs.io/en/stable/common_issues.html
     if sys.platform == "win32" or sys.platform == "cygwin":
         return
-    if sys.platform == "darwin" or sys.platform == "linux":
-        if os.getuid() != 0:
-            return
+
+    if os.getuid() != 0:
+        return
+
     logger.warning(
         "Running pip as the 'root' user can result in broken permissions and "
         "conflicting behaviour with the system package manager. "