Error prone env naming

[PierreJeanjacquot]

The file name writting is not consitent accross IEXEC_DATASET_FILENAME & IEXEC_INPUT_FILE_NAME_1 wich is error prone.

iexec-common/src/main/java/com/iexec/common/utils/IexecEnvUtils.java

Line 42 in e73ec20

public static final String IEXEC_DATASET_FILENAME = "IEXEC_DATASET_FILENAME";

iexec-common/src/main/java/com/iexec/common/utils/IexecEnvUtils.java

Line 51 in e73ec20

public static final String IEXEC_INPUT_FILE_NAME_PREFIX = "IEXEC_INPUT_FILE_NAME_";

We should choose one of the writtings and make it consistent across all the variables.
It could be useful to alias the replaced writting to keep backward compatibility.