Disallow var syntax in string interpolation

[c42f]

The var"##" syntax should be disabled in string interpolation.

Also work around a subtle problem with command interpolation's use of Meta.parse in non-greedy mode, combined with the new var syntax which would cause Base.shell_parse("\"(\$var)\"") to miss the trailing ) character.

This is a quick fix for a bit of fallout from #32408 which affects PyCall's build script, among other things.

Fixes #32951

[c42f]

Right I've sorted those problems out!

I also added a workaround to preserve the behavior of Base.shell_parse (and hence @cmd interpolation) with $var interpolation. This is ugly but the alternative is a non backward compatible change to the way that @cmd handles $ interpolation in general.

I think we should change the way $ interpolation works in cmd, but I don't want to couple that breaking change with getting this bug fixed.

[c42f]

CI errors look unrelated (win64 — temp files; freebsd — an internal build slave error).

[c42f]

@JeffBezanson can you live with the ugly shell_parse compatibility workaround I've got here? If so I'd like to merge this and come back to the inconsistency between cmd interpolation vs string interpolation another time. (Edit: to be clear, this inconsistency is nothing new.)

[MikeInnes]

This bug breaks both Juno and the CUDA stack, so it'd be great to get the fix in quickly.

[chriscoey]

bump