python executable not detected outside of pipenv project

My system python is installed via homebrew/miniconda:
```
$ which python
/usr/local/Caskroom/miniconda/base/bin/python
```
When the python executable setting is left as default `python` PyLS spins up from the base environment fine.

If I set up a project with pipenv and use the suggested `$PIPENV_PATH/bin/python` then it spins up PyLS in the virtual environment and works fine for that project. However, outside of the project the python executable cannot be found.

From reviewing the code it looks like outside of a pipenv project the python path becomes `/bin/python` which is not where my system python lives. It needs to fallback to just `python`.

----
I can see that `$PIPENV_PATH` gets replaced with the output of `pipenv --venv`, however you could use `pipenv --py` to get the full path to the interpreter (or just append `/bin/python` as it's always that). In that case the executable setting won't be needed for pipenv, so it can just remain as the default `python` or be set as an absolute path if needed.

----
N.B As a workaround I tried to symlink `/bin/python` to my miniconda python. However on MacOS SIP prevents this. I'd rather not have to disable it.

该 issue 涉及可执行文件设置和 Pipenv 集成；首先跟踪不存在 Pipenv 项目时 `$PIPENV_PATH` 的展开方式。将报告的 `pipenv --venv` 行为与 `pipenv --py` 进行比较，然后验证默认的 `python` 在 Pipenv 项目内外都能正常工作。

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