envsubst - Fix when more than 2 arguments
envsubst - Fix when more than 2 arguments

The previous code:

    files_count=$((  + 1 ))

Is effectively the same as:

    files_count=1

I suspect the double space meant originaly it was:

    files_count=$(( files_count + 1 ))

Somehow an editing error occurred and the `files_count` was removed.
The result is the first argument is indexed at 0 (from above) all
subsequent values are indexed at 1 with each later argument overwriting
the previous. This means only the first and last argument are actually
processed.

This fixes the code so that all arguments are processed.

GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community,GoogleCloudPlatform/cloud-builders-community