Avoid a specific case where conda env update hangs. (conda#11818)

When using an environment.yml that asks pip to clone a git repo and
that path exists, pip will ask for user input. As we are not in a
context to provide such, tell pip what to do so it doesn't hang on
waiting for user input.

conda_env/installers/pip.py

@@ -48,7 +48,8 @@ def _pip_install_via_requirements(prefix, specs, args, *_, **kwargs):
         requirements.write('\n'.join(specs))
         requirements.close()
         # pip command line...
-        pip_cmd = ['install', '-U', '-r', requirements.name]
+        # see https://pip.pypa.io/en/stable/cli/pip/#exists-action-option
+        pip_cmd = ['install', '-U', '-r', requirements.name, '--exists-action=b']
         stdout, stderr = pip_subprocess(pip_cmd, prefix, cwd=pip_workdir)
     finally:
         # Win/Appveyor does not like it if we use context manager + delete=True.

news/11818-conda-hang

@@ -0,0 +1,21 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* `conda env update` would ask for user input and hang when working with pip
+installs of git repos and the repo was previously checked out. Tell pip not to
+ask for user input for that case.  (#11818)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>