[bootstrap/conda] Pin conda=4.6 to fix "conda update" issues on base env

bob/devtools/bootstrap.py

@@ -386,7 +386,7 @@ if __name__ == '__main__':
   with open(condarc, 'wt') as f:
     f.write(_BASE_CONDARC)
 
-  conda_version = '4.16'
+  conda_version = '4.6'
   conda_build_version = '3.16'
   conda_verify_version = '3'
 

conda/meta.yaml

@@ -30,7 +30,7 @@ requirements:
     - setuptools
     - click >=7
     - click-plugins
-    - conda=4.16
+    - conda=4.6
     - conda-build=3.16
     - conda-verify=3
     - certifi