Fix regex for getting CONFIG_VARIANT from CONFIG_NAME

[punchagan]

The previous regex would parse a name like 5.1.0+trunk+fabbing+prefetching_like_414 to 5.1.0+trunk\n414. This commit makes sure we only look at digits at the begnning of the config name.

NOTE The run_all_custom.sh script needs to be copied onto the bench machines.

[punchagan]

This would fix broken benchmark runs like this

[shakthimaan]

Thanks!