Add --disable-dev-shm-usage to the CI Chrome arguments

Chrome keeps its shared memory in /dev/shm, which is 64 MB in the CI services. The Umami performance test runs out of it and the tab crashes, so the scheduled Performance tests job has failed every time it has run since 31 July: 20 runs, 20 failures in the last 24 scheduled pipelines.

The Nightwatch job already passes --disable-dev-shm-usage, on line 302 of this file. This adds it to line 87.

Locally on selenium/standalone-chrome:133.0 with /dev/shm at 64 MB, three runs per cell: the test passes with the flag, both idle and under 8 concurrent sessions, and fails without it in both. Thresholds are unchanged.