_should_cache is only called by _get_cache_dir.
In pip install mode, _get_cache_dir is never called when
check_binary_allowed returns False because in that case
should_build_for_install_command has returned False before
and the build was skipped.
In pip wheel mode, check_binary_allowed always returns True
(because it is not passed to the build function).
So _should_cache can use _always_true for check_binary_allowed.
*Alternative*
Alternatively, we could have passed check_binary_allowed
to build in pip wheel mode. The only difference is that wheels built
locally from *legacy* packages would then not be cached,
when pip wheel is used with --no-binary.
Separately test should_build_for_wheel_command
and should_buid_for_install_command.
The tests are simpler and this open the door
for reasoning about both functions independently.
