Default packages fail to install if `PIP_REQUIRE_VIRTUALENV` is set

When installing a new Python version, it failed when attempting to install default packages:

```
Installed Python-3.11.4 to /Users/myusername/.asdf/installs/python/3.11.4

Installing default python packages...ERROR: Could not find an activated virtualenv (required).
```

This is because I have `PIP_REQUIRE_VIRTUALENV` set to `true` in my shell. Looking at the install command, it uses `pip` as such:

https://github.com/asdf-community/asdf-python/blob/8505457c6612514688ba473386d2063d4c83bb54/bin/install#L38

According to [this Stack Overflow answer](https://stackoverflow.com/a/30515378), I believe this command should include the `--isolated` flag. In my case, adding the flag works:

```
$ pip install -U -r ~/.default-python-packages
ERROR: Could not find an activated virtualenv (required).

$ pip install -U -r ~/.default-python-packages --isolated
Requirement already satisfied: ...
```