Change target folder to /target and move on to version 0.13

I tested this with the expected results on a local workspace with 9 contracts. In all cases, no local ./target folder was created. I.e. the optimizer does not mess with the local build directory.

No target cache set (not recommended but still solid)

Run 1: 5:58,49 total (the first one created the registry_cache cache. The following ones are a minute faster)
Run 2: 4:55,18 total
Run 3: 5:00,33 total
Run 4: 5:19,70 total

Target mounted (--mount type=volume,source="$(basename "$(pwd)")_cache",target=/target)

Run 1: 5:29,11 total
Run 2: 27,760 total (huge win here, as expected)
Run 3: 28,324 total
Run 4: 29,328 total

CosmWasm / optimizer

Change target folder to /target and move on to version 0.13 #124