units: fix build with enableCurrenciesUpdater=false; pythonPackages=null;

Units allows to build it without a python dependency by setting {
enableCurrenciesUpdater=false; pythonPackages=null; }.  Unfortunately
this feature is currently broken due to two problems:

1. The `pythonEnv` string is part of the builder environment, so it is
   not evaluated lazily.  This means that `pythonPackages==null` will
   always cause eval to fail.

2. `pythonEnv` is used unconditionally in an antiquotation in the
   `prePatch` phase; if it is null this will fail.

Let's fix these so we can build a pythonless "units" package.

This is helpful when cross-compiling, because right now a lot of
python packages (especially python-cryptography) fail to
cross-compile.

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