show math.equation: set align(..) shall not break alignment points

[Leedehai]

Fixes #3973.

Note that the alignment points (i.e. the &s) in cases() do not have this left/right-alternating behavior, as specifically checked by this existing test (note the quad there):

--- math-cases ---
$ f(x, y) := cases(
  1 quad &"if" (x dot y)/2 <= 0,
  2 &"if" x divides 2,
  3 &"if" x in NN,
  4 &"else",
) $

If we unconditionally applied the left/right alternating behavior, then the output would be:
which is undesired.

Therefore, we should apply the alternating behavior in (almost) all situation but except for cases, where alignment points do not alternate.

[laurmaedje]

Thanks!