Meson: Search shell executable only once
Currently each meson custom_target() using sh search for the binary itself, meaning it gets searched multiple times. Search it just once and store result to an executable variable, which is then used in all places where its needed.
Rebased (now applies without #43362)
Currently each meson custom_target() using sh search for the binary itself, meaning it gets searched multiple times. Search it just once and store result to an executable variable, which is then used in all places where its needed.