Make functional tests depend on nix binary so they auto recompile

With this I'm able to do a fresh config + meson test with all dependencies
correctly propagated.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

tests/functional/meson.build

@@ -223,6 +223,18 @@ subdir('local-overlay-store')
 foreach suite : suites
   workdir = suite['workdir']
   suite_name = suite['name']
+  # This is workaround until [1] is resolved. When building in a devshell
+  # as a subproject we want the tests to depend on the nix build target, so
+  # that it gets automatically rebuilt.
+  # However, when the functional test suite is built separately (via componentized
+  # builds or in NixOS tests) we can't depend on the nix executable, since it's
+  # an external program. The following is a simple heuristic that suffices for now.
+  # [1]: https://github.com/mesonbuild/meson/issues/13877
+  deps = suite['deps']
+  if meson.is_subproject()
+    nix_subproject = subproject('nix')
+    deps += [ nix ] + nix_subproject.get_variable('nix_symlinks_targets')
+  endif
   foreach script : suite['tests']
     # Turns, e.g., `tests/functional/flakes/show.sh` into a Meson test target called
     # `functional-flakes-show`.
@@ -252,7 +264,7 @@ foreach suite : suites
       # them more time than the default of 30 seconds.
       timeout : 300,
       # Used for target dependency/ordering tracking, not adding compiler flags or anything.
-      depends : suite['deps'],
+      depends : deps,
       workdir : workdir,
     )
   endforeach