Just: check the fixtures against the just the code will run

`forward_command` honours JUST_EXECUTABLE while these tests went to PATH, so
pinning a `just` would have had the fixtures agree with one binary and the
code run another. Resolving the module's own setting also makes the tests
reachable under `bazel test`, whose sandbox offers a fixed PATH that no
`just` is on, so they skipped there on every machine.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/test_forward_command.py

@@ -308,7 +308,11 @@ def test_two_roots_sharing_a_comment_still_both_run_if_they_do_different_work(se
         self.assertEqual([justfile for justfile, _ in found], [self.inner, self.outer])
 
 
-JUST = shutil.which("just")
+# Resolve the same binary `forward_command` will run: it honours JUST_EXECUTABLE, so a
+# pinned `just` would otherwise have these fixtures checked against a different binary
+# than the code uses. `which` covers both spellings, returning an explicit path as given
+# and looking a bare name up on PATH.
+JUST = shutil.which(forward_command.JUST)
 
 # The justfile below uses every construct the fixtures above model, so that a dump of it
 # can be checked against them.