Fix bats-preprocess not found in bats-gather-tests

when PATH is modified and multiple files are loaded.

lib/bats-core/preprocessing.bash

diff --git a/lib/bats-core/preprocessing.bash b/lib/bats-core/preprocessing.bash
index 5d9a765..7e15b5a 100644
--- a/lib/bats-core/preprocessing.bash
+++ b/lib/bats-core/preprocessing.bash
@@ -9,7 +9,7 @@
   # export to make it visible to bats_evaluate_preprocessed_source
   # since the latter runs in bats-exec-test's bash while this runs in bats-exec-file's
   export BATS_TEST_SOURCE="${BATS_TMPNAME}.src"
-  CHECK_BATS_COMMENT_COMMANDS=1 bats-preprocess "$BATS_TEST_FILENAME" >"$BATS_TEST_SOURCE"
+  CHECK_BATS_COMMENT_COMMANDS=1 "$BATS_ROOT/libexec/bats-core/bats-preprocess" "$BATS_TEST_FILENAME" >"$BATS_TEST_SOURCE"
 }
 
 bats_evaluate_preprocessed_source() {