Unit File Load Path

Unit files are loaded from a set of paths determined during compilation, described in the two tables below. Unit files found in directories listed earlier override files with the same name in directories lower in the list.

When the variable $SYSTEMD_UNIT_PATH is set, the contents of this variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.

To set the environment of the systemd --system bus, we need to look at man systemd-system.conf. Then set values in these files to affect the system bus:

/etc/systemd/system.conf
/etc/systemd/system.conf.d/*.conf   <-- Install your change here
/run/systemd/system.conf.d/*.conf
/lib/systemd/system.conf.d/*.conf

In terms of what to put in this configuration, the man page descirbes:

ManagerEnvironment=
    Takes the same arguments as DefaultEnvironment=, see above. Sets 
    environment variables just for the manager process itself. In contrast 
    to user managers, these variables are not inherited by processes spawned 
    by the system manager, use DefaultEnvironment= for that. Note that these
    variables are merged into the existing environment block. In particular, 
    in case of the system manager, this includes variables set by the kernel 
    based on the kernel command line.

Therefore install this file as /etc/systemd/system.conf.d/mypath.conf:

[Manager]
ManagerEnvironment="SYSTEMD_UNIT_PATH=/my/path:"