Revert "Pass --local-engine* flags from dev/bots/test.dart down to `pub test` via env variables (#71721)" (#72141)

This reverts commit 4300226a.

Reason for revert: The original PR correctly passes down the local
engine flags to subprocesses of the tests. Though this causes issues
because some tests assume a specific target but do not modify the
value for `--local-engine` (which is set to `host_debug`) to reflect
that.

See more discussion here:

    https://github.com/flutter/flutter/pull/71721#issuecomment-743218994