pythonPackages.pip: make reproducible (#102222)

The previous attempt wasn't covering all of the bases. It relied on
invoking that pip-install-hook, and didn't apply to pip itself.

The core issue is that the generated .pyc files embed some of the
temporary paths, which are randomly generated. See
https://r13y.com/diff/bf8c3ca3148ebff9ecf41f294cc60b9f209c006d49699e356969ff32d736f1c6-8806a7cca91fdd300e48736bfcd57c4d0b54c1cc2fd61609f35143170862b59c.html

In this new attempt, the approach is to patch the TempFile
implementation directly, so that it creates stable temporary
directories. We also assume that if SOURCE_DATE_EPOCH is set, we are in
a scenario where reproducible builds are desirable and enter that
branch.

See also https://github.com/pypa/pip/issues/7808

pkgs/development/python-modules/pip/default.nix

@@ -26,7 +26,7 @@ buildPythonPackage rec {
   };
 
   # Remove when solved https://github.com/NixOS/nixpkgs/issues/81441
-  # Also update pkgs/development/interpreters/python/hooks/pip-install-hook.sh accordingly
+  # See also https://github.com/pypa/pip/issues/7808
   patches = [ ./reproducible.patch ];
 
   nativeBuildInputs = [ bootstrapped-pip ];