Fix path for Cargo.toml

Index: tests/testsuite/preprocessor/failing_preprocessor/book.toml
--- tests/testsuite/preprocessor/failing_preprocessor/book.toml.orig
+++ tests/testsuite/preprocessor/failing_preprocessor/book.toml
@@ -1,4 +1,4 @@
 [preprocessor.nop-preprocessor]
-command = "cargo run --quiet --example nop-preprocessor --"
+command = "cargo run --quiet --example nop-preprocessor --manifest-path ${WRKDIST}/Cargo.toml"
 blow-up = true
 
