🐛 Fix append_deactivate_text for venv

From Python 3.10, the `VIRTUAL_ENV_PROMPT` environment variable is also
set and unset in the `activate` script. Since our current (obviously rather
fragile) approach to adding text to the `deactivate` function in the `activate`
script relies on replacing `VIRTUAL_ENV`, this would break and give the
following error:

```
deactivate:31: command not found: _PROMPT
```

when activating or deactivating the environment.

Here we switch to replacing the `deactivate () {` line in the `activate` script
that. That should be more robust unless the function definition syntax is
changed.

aiida_project/commands/main.py

@@ -24,7 +24,10 @@
 eval "$(_VERDI_COMPLETE={shell}_source verdi)"
 """
 
-DEACTIVATE_AIIDA_SH = "unset AIIDA_PATH"
+DEACTIVATE_AIIDA_SH = """
+# Added by `aiida-project`
+unset AIIDA_PATH
+"""
 
 
 app = typer.Typer(pretty_exceptions_show_locals=False)

aiida_project/project/venv.py

@@ -38,8 +38,8 @@ def append_deactivate_text(self, text):
         with Path(self.venv_path, "bin", "activate").open("w") as handle:
             handle.write(
                 contents.replace(
-                    "unset VIRTUAL_ENV",
-                    f"unset VIRTUAL_ENV\n{text}\n",
+                    "deactivate () {",
+                    "deactivate () {" + f"\n{text}\n",
                 )
             )