Don't use ${project} in system_perf.yml

In SERVER-25348 we have introduced an expansion ${project} in system_perf.yml. Turns out it is a builtin Evergreen expansion. So we should use something else.

AFAIU we could simply remove the setting of this expansion, and use the value from the builtin expansion? We set it to "dsi", "dsi-v3.6", etc... and the builtin would be "sys-perf", "sys-perf-3.6"...