Add build to deps along with asv

We are hitting odd

    Run asv run --show-stderr --python 3.8 HEAD^-1
    Couldn't load asv.plugins._mamba_helpers because
    No module named 'libmambapy'

which was presumably the

   airspeed-velocity/asv#1385

which was addressed by

   airspeed-velocity/asv#1387

which was released in 0.6.3 but we are installing it

    Collecting asv!=0.6.2 (from asv[virtualenv]!=0.6.2)
      Downloading asv-0.6.3-cp38-cp38-manylinux_2_17_x86_64.manylinux2014_x86_64.whl.metadata (3.8 kB)

and still hitting it. Trying now with explicit "build" in depends

tox.ini

@@ -28,6 +28,7 @@ commands =
 skip_install = True
 deps =
     asv[virtualenv] != 0.6.2
+    build
 commands =
     asv run {posargs} HEAD^1..HEAD