BUG: Add `MAMBARC` support to `mamba`

[HaoZeke]

~Cannot be reviewed until #1372 is in~

Closes #1358. Also explicitly removes conda-forge as a default for mamba. Users are free to either:

• Specify it in their .condarc and use export MAMBARC=$HOME/.condarc or (better)
• Explicitly provide an environment.yml or set the corresponding variable in asv.conf.json

The documentation has also been updated to reflect the same.

[mattip]

Does this need a rebase now that #1372 is in? Does this need a specific test?

[HaoZeke]

Does this need a rebase now that #1372 is in? Does this need a specific test?

@mattip, I've rebased and added more tests covering all the additional features for the mamba solver, should be good to go.

test/test_environment_bench.py::test_asv_mamba[with_conda_forge] PASSED  [ 50%]
test/test_environment_bench.py::test_asv_mamba[empty_conda_channels] PASSED [ 62%]
test/test_environment_bench.py::test_asv_mamba_condarc[with_proper_condarc_and_mambarc] PASSED [ 75%]
test/test_environment_bench.py::test_asv_mamba_condarc[with_condarc_but_no_mambarc] PASSED [ 87%]
test/test_environment_bench.py::test_asv_mamba_condarc[without_condarc_and_mambarc] PASSED [100%]

[mattip]

CI is not happy. It would be nice to have at least one windows (and maybe one macos) CI run to prove the code works on those platforms as well, since path parsing differs.

[HaoZeke]

@mattip I think this now ready.

[mattip]

thanks @HaoZeke