meson: Call find_program() once for GJS

Avoiding multiple calls to find_program()
for the generated test scripts.
1 job for wip/mlundblad/single-gjs-find-program in 11 minutes and 23 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #1291157
flatpak
flatpak

00:11:23