python3.pkgs.pip: 20.1.1 -> 20.2.4

Reproducible builds of pyproject projects using pip is resolved.

Fixes https://github.com/pypa/pip/issues/7808
Fixes https://github.com/NixOS/nixpkgs/issues/81441

The more recent c409f69480 caused trouble
with pyproject troubles and had to be reverted anyway.
https://github.com/NixOS/nixpkgs/pull/102222#issuecomment-722380794

Revert "pythonPackages.pip: make reproducible (#102222)"

This reverts commit c409f69480.

Revert "python3Packages.pip: allow setting reproducible temporary directory via NIX_PIP_INSTALL_TMPDIR"

This reverts commit aedbade43e.

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

@@ -11,9 +11,7 @@ pipInstallPhase() {
     export PYTHONPATH="$out/@pythonSitePackages@:$PYTHONPATH"
 
     pushd dist || return 1
-    mkdir tmpbuild
-    @pythonInterpreter@ -m pip install ./*.whl --no-index --prefix="$out" --no-cache $pipInstallFlags
-    rm -rf tmpbuild
+    @pythonInterpreter@ -m pip install ./*.whl --no-index --prefix="$out" --no-cache $pipInstallFlags --build tmpbuild
     popd || return 1
 
     runHook postInstall