shared/bus-unit-util: tweak bus_append_exec_command to use Ex prop only if necessary

shared/bus-unit-util: tweak bus_append_exec_command to use Ex prop only if necessary

This changes little in behaviour, the conceptual part is more important. The
non-Ex variant is the actual name on the command line, and we should use the
non-Ex D-Bus property too, if it works. This increases compatibility with old
versions. But the code was mostly doing the right thing. Even the tests tested
the right thing.

Follow-up for b3d593673c5b8b0b7d781fd26ab2062ca6e7dbdb and
898fc00e794d714e2f01409bef440d910c22502a.

The test is simplified by taking advantage of the fact that both names
on the commandline are supposed to behave identically.

Partially resolves https://github.com/systemd/systemd/issues/37174.