rhg: Add environment variables for fallback configuration
For the `rust-tests.py --rhg` we want every `hg` command in tests
to run `rhg` with fallback enabled, but other methods of setting
configuration are limited or disruptive as discussed in code comment.
Differential Revision: https://phab.mercurial-scm.org/D10186
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true