Fix benchmark CI

[dcherian]

What is your issue?

It's failing during setup: https://github.com/pydata/xarray/runs/6424624397?check_suite_focus=true

· Discovering benchmarks
·· Uninstalling from conda-py3.8-bottleneck-dask-distributed-flox-netcdf4-numpy-numpy_groupies-pandas-scipy-sparse
·· Building dd20d07f for conda-py3.8-bottleneck-dask-distributed-flox-netcdf4-numpy-numpy_groupies-pandas-scipy-sparse
·· Error running /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/bin/python -mpip wheel --no-deps --no-index -w /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/asv-build-cache/dd20d07f4057a9e29222ca132c36cbaaf3fbb242 /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project (exit status 1)
   STDOUT -------->
   Processing /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project
   STDERR -------->
   ERROR: Some build dependencies for file:///home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project are missing: 'setuptools_scm[toml]>=3.4', 'setuptools_scm_git_archive'.

·· Failed: trying different commit/environment
·· Uninstalling from conda-py3.8-bottleneck-dask-distributed-flox-netcdf4-numpy-numpy_groupies-pandas-scipy-sparse
·· Building c34ef8a6 for conda-py3.8-bottleneck-dask-distributed-flox-netcdf4-numpy-numpy_groupies-pandas-scipy-sparse
·· Error running /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/bin/python -mpip wheel --no-deps --no-index -w /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/asv-build-cache/c34ef8a60227720724e90aa11a6266c0026a812a /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project (exit status 1)
   STDOUT -------->
   Processing /home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project
   STDERR -------->
   ERROR: Some build dependencies for file:///home/runner/work/xarray/xarray/asv_bench/.asv/env/e8ce5703538597037a298414451d04d2/project are missing: 'setuptools_scm[toml]>=3.4', 'setuptools_scm_git_archive'.