Expose some topgrade-related environment variables when running custom step commands

[Jisu-Woniu]

I want to suggest some general feature

The built-in steps of topgrade allow for different behaviors when running with -y or -c args, but custom commands cannot handle this without checking for the parent process.

I suggest topgrade expose some environment variables like TOPGRADE_YES=1 or TOPGRADE_CLEANUP=1 when running with -y/--yes or -c/--cleanup, so we can check the flags with a single if in shell scripts.

[Jisu-Woniu]

Also, -n/--dry-run can be exposed as TOPGRADE_DRY_RUN=1