CMake adds the imported mujoco target's include dir as -isystem, so with
system_headers sloppiness ccache stopped hashing the public MuJoCo headers.
A header-only change (new mjData field, new enum value) then went undetected
and ccache reused objects compiled against the old struct layout, yielding an
ABI-mismatched binding: wrong field offsets (garbage in struct/pickle tests),
stale mjENABLESTRING/mjNENABLE, and mjSpec-vs-mjData signature mismatches.
Removing system_headers makes ccache hash these headers and recompile on
change. The mtime/ctime flags stay (they absorb pip's per-run temp-dir churn
without affecting content detection).
Kevin Zakka
ffd95cf656