[LESS] Less 3.8.1 doesn't let you negate variables inside calc

[mikejf-pr]

Less had a number of variables which use the following notation:

@ribbonImageOffset: calc(-@ribbonImageMargin - @ribbonTriangleSize);

At the moment, use of code like

@a: -4px;
div {
	margin-right: calc(-@a + 4em);
}

in less 3.8.1 will result in

div {
  margin-right: calc(--4px + 4em);
}