CircleCI tests with prerelease (nightlies) are failing due to conda timeout

[tswast]

Stuck on "solving environment"

+++ export CONDA_PREFIX=/opt/conda/envs/test-environment
+++ CONDA_PREFIX=/opt/conda/envs/test-environment
+++ export CONDA_SHLVL=1
+++ CONDA_SHLVL=1
+++ export CONDA_DEFAULT_ENV=test-environment
+++ CONDA_DEFAULT_ENV=test-environment
+++ export CONDA_PROMPT_MODIFIER=
+++ CONDA_PROMPT_MODIFIER=
+++ export CONDA_EXE=/opt/conda/bin/conda
+++ CONDA_EXE=/opt/conda/bin/conda
+++ export _CE_M=
+++ _CE_M=
+++ export _CE_CONDA=
+++ _CE_CONDA=
+++ export CONDA_PYTHON_EXE=/opt/conda/bin/python
+++ CONDA_PYTHON_EXE=/opt/conda/bin/python
++ __conda_hashr
++ '[' -n '' ']'
++ '[' -n '' ']'
++ hash -r
+ REQ=ci/requirements-3.9-NIGHTLY
+ conda install -q --file ci/requirements-3.9-NIGHTLY.conda
+ local cmd=install
+ case "$cmd" in
+ __conda_exe install -q --file ci/requirements-3.9-NIGHTLY.conda
+ __add_sys_prefix_to_path
+ '[' -n '' ']'
++ dirname /opt/conda/bin/conda
+ SYSP=/opt/conda/bin
++ dirname /opt/conda/bin
+ SYSP=/opt/conda
+ '[' -n '' ']'
+ PATH=/opt/conda/bin:/opt/conda/envs/test-environment/bin:/opt/conda/condabin:/opt/conda/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+ export PATH
+ /opt/conda/bin/conda install -q --file ci/requirements-3.9-NIGHTLY.conda
Collecting package metadata (current_repodata.json): ...working... done
Solving environment: ...working... failed with initial frozen solve. Retrying with flexible solve.
Solving environment: ...working... failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): ...working... done
Solving environment: ...working... /opt/conda/etc/profile.d/conda.sh: line 35:   799 Killed                  "$CONDA_EXE" $_CE_M $_CE_CONDA "$@"
+ return

Exited with code exit status 137
CircleCI received exit code 137

https://app.circleci.com/pipelines/github/googleapis/python-bigquery-pandas/150/workflows/f1bfd01f-dd32-42bc-90d4-4a11c26046ec/jobs/1108

Maybe switch to mamba?

[tswast]

Alternatively, we could just depend on the pip prerelease session we just added and remove the CircleCI build. We'll still need some conda builds that (1) test with pinned dependencies and (2) test with latest available dependencies