devel/mustach: Update to 1.2.6

In addition to the upstream changes below, the package now also includes
the cJSON backend (and dependency).

Fix:
- improve naming (#42)
- magical spaces in recursive partials (#43)
- installation when tool isn't built
- correct detection of falsey values (#45)

Minor:
- update to newer reference tests

https://gitlab.com/jobol/mustach/-/releases/1.2.6

(sjmulder)