backends: restore shlex quoting of MESONINTROSPECT

The type of quoting was changed in 522392e to one that is suitable for
use with cmd.exe on Windows. However, the documentation states that the
type of quoting in MESONINTROSPECT is compatible with shlex.split() and
elsewhere in the code, the same variable is still quoted with
shlex.quote(). As mostly identified in #12148, there are a few choices:
1. Use shlex.quote() consistently and support Python but not cmd.exe.
2. Use join_args and support cmd.exe but not Python.
3. Use join_args and support splitting through the mesonbuild Python library.

This commit implements the first option and reverts part of 522392e.

Regression testing is implemented in #12115.

Fixes #12148

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