restore_cache: support providing --key-fallback multiple times

it would be nice to be able to specify `--key-fallback` multiple times and have all fallback keys tried in the order specified

## Affected builder image

`gcr.io/cloud-builders-community/restore_cache`

## Expected Behavior

when `--key-fallback` is specified multiple times, all keys should be tried in the order specified

## Actual Behavior

when `--key-fallback` is specified multiple times, only last value is used