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/interpreters/python/hooks/pip-install-hook.sh

@@ -12,7 +12,7 @@ pipInstallPhase() {
 
     pushd dist || return 1
     mkdir tmpbuild
-    NIX_PIP_INSTALL_TMPDIR=tmpbuild @pythonInterpreter@ -m pip install ./*.whl --no-index --prefix="$out" --no-cache $pipInstallFlags
+    @pythonInterpreter@ -m pip install ./*.whl --no-index --prefix="$out" --no-cache $pipInstallFlags
     rm -rf tmpbuild
     popd || return 1