core: fix memleak in bus_exec_context_set_transient_property (#5143)

Fixes:
```sh
systemd-run --property EnvironmentFile=/some/environment/file /bin/sleep 30
```
```
 23 bytes in 1 blocks are definitely lost in loss record 1 of 7
    at 0x4C2DB9D: malloc (vg_replace_malloc.c:299)
    by 0x4E85488: malloc_multiply (alloc-util.h:70)
    by 0x4E85F19: strjoin_real (string-util.c:252)
    by 0x1AF741: bus_exec_context_set_transient_property (dbus-execute.c:1418)
    by 0x1A907C: bus_service_set_property (dbus-service.c:330)
    by 0x1A66DD: bus_unit_set_properties (dbus-unit.c:1456)
    by 0x19CF93: transient_unit_from_message (dbus-manager.c:892)
    by 0x19D388: method_start_transient_unit (dbus-manager.c:980)
    by 0x4F60544: method_callbacks_run (bus-objects.c:418)
    by 0x4F62D9D: object_find_and_run (bus-objects.c:1255)
    by 0x4F633CE: bus_process_object (bus-objects.c:1371)
    by 0x4F2CE1D: process_message (sd-bus.c:2563)
```
Closes: #5142

(cherry picked from commit 9b531f04)