Disable job control inside command substitutions

This disables job control inside command substitutions. Prior to this
change, a cmdsub might get its own process group. This caused it to fail
to cancel loops properly. For example:

    while true ; echo (sleep 5) ; end

could not be control-C cancelled, because the signal would go to sleep,
and so the loop would continue on. The simplest way to fix this is to
match other shells and not use job control in cmdsubs.

Related is #1362

CHANGELOG.rst

@@ -29,6 +29,7 @@ Deprecations and removed features
 
 - ``$status`` is now forbidden as a command, to prevent a surprisingly common error among new users: Running ``if $status`` (:issue:`8171`). This applies *only* to ``$status``, other variables are still allowed.
 - ``set --query`` now returns a falsy status of 255 if given no variable names. This means ``if set -q $foo`` will not enter the if-block if ``$foo`` is empty or unset. To restore the previous behavior you would use something like ``if not set -q foo; or set -q $foo``. We do not expect anyone to have used this on purpose, any places this happens are almost certainly buggy (:issue:`8214`).
+- Command substitutions no longer respect job control, instead running inside fish's own process group. This more closely matches other shells, and improves :kbd:`Control-C` reliability inside a command substitution.
 
 Scripting improvements
 ----------------------