migrator: fix default entrypoint behaviour

Created by: bobheadxi

I broke this in https://github.com/sourcegraph/sourcegraph/pull/33758 by not correctly using the modified args []string provided See: https://github.com/sourcegraph/customer/issues/845

Test plan

Before:

➜  sourcegraph git:(main) go run ./cmd/migrator
NAME:
   migrator - Validates and runs schema migrations

USAGE:
   migrator command [command options] [arguments...]

COMMANDS:
   up        Apply all migrations
   upto      Ensure a given migration has been applied - may apply dependency migrations
   downto    Revert any applied migrations that are children of the given targets - this effectively "resets" the schmea to the target version
   validate  Validate the current schema
   add-log   Add an entry to the migration log
   help, h   Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help (default: false)

After:

➜  sourcegraph git:(fix-migrator-default-cmd) go run ./cmd/migrator                                      
error: databases frontend and codeintel must be distinct, but both target postgres://127.0.0.1:5432
exit status 1