nixos/tests/prometheus-2.nix: increase diskSize of the store machine

This is to fix the following error in the test on aarch64-linux:

store# [  126.911144] thanos[739]: level=error ts=2019-06-16T14:00:26.59870538Z caller=main.go:182 msg="running command failed" err="error executing compaction: first pass of downsampling failed: create dir: mkdir /var/lib/thanos-compact/downsample: no space left on device"
store# [  126.942655] systemd[1]: thanos-compact.service: Main process exited, code=exited, status=1/FAILURE