Run Package Control settings helper in isolated mode (#308)

This commit fixes following scenario:

`install_package_control.sh` adds the packages listed by name in
`extra-packages` to Package Control's settings with `python3 -B -`. A script
read from stdin gets the current directory at the start of `sys.path`, and
that directory is usually the package checkout. So a module in the package's
root that has the name of a standard library module gets imported instead of
it.

In sublimelsp/LSP-elm, `types.py` breaks `import json` on macOS 
(Homebrew Python 3.15), since `enum` imports `types`:

```
  File "/Users/runner/work/LSP-elm/LSP-elm/types.py", line 3, in <module>
    from LSP.protocol import DocumentUri
ModuleNotFoundError: No module named 'LSP'
```

Ubuntu and Windows pass, probably because their Python has already imported
`types` at startup.

`-I` (isolated mode) leaves the current directory and user site-packages off
`sys.path` and ignores `PYTHON*` environment variables. The helper only needs
the standard library. The PowerShell script doesn't run Python here.

Author: rchl

actions/setup/install_package_control.sh

@@ -59,7 +59,9 @@ fi
 if [ -n "$UNITTESTING_PC_PACKAGES" ]; then
     echo Adding packages to be installed by Package Control:
     printf '%s' "$UNITTESTING_PC_PACKAGES" | sed 's/^/  /'
-    python3 -B - "$STP/User/Package Control.sublime-settings" <<'EOF'
+    # -I keeps the current directory, usually the package checkout, off sys.path, so that its modules can't shadow the
+    # standard library, e.g. a `types.py` breaking `import json`.
+    python3 -I -B - "$STP/User/Package Control.sublime-settings" <<'EOF'
 import json
 import os
 import sys