x/benchmarks, x/build: gVisor startup benchmark doesn't work on builders

[mknyszek]

After reenabling the gVisor benchmarks, I ran into an issue with the 'startup' benchmark failing to write to /var/run/runsc which shouldn't be necessary, and doesn't seem to be happening locally. I'm not sure what's causing it, but it seems related to the runsc run command in particular, as the sub-benchmarks work just fine (and use runsc do).