Use -I with ensurepip

The -s flag assures that nothing can be installed to user site-packages
but doesn't keep ensurepip from looking there for Pip.
If Pip is installed in the user site-packages directory, pip won't be installed
for the newly built python -- and its shim won't be created.
-I makes the build install Pip in any case.
The user site-packages installation will override it --
but we'll have the shim at least.

plugins/python-build/bin/python-build

@@ -2025,7 +2025,7 @@ build_package_ensurepip() {
     ensurepip_opts="--altinstall"
   fi
   # FIXME: `--altinstall` with `get-pip.py`
-  "$PYTHON_BIN" -s -m ensurepip ${ensurepip_opts} 1>/dev/null 2>&1 || build_package_get_pip "$@" || return 1
+  "$PYTHON_BIN" -I -m ensurepip ${ensurepip_opts} 1>/dev/null 2>&1 || build_package_get_pip "$@" || return 1
   build_package_symlink_version_suffix
 }