If no manual stamp is found, then run cargo clean anyway

Since the beta and stable channels are not currently saving their
caches, we run `cargo clean` if no file is found.

Author: jtgeibel

script/ci/cargo-clean-on-new-rustc-version.sh

@@ -11,10 +11,15 @@ if [ -f $manual_stamp_file ]; then
     else
         echo "A clean build has been requested, running cargo clean"
         cargo clean
-        # The target/ directory is now gone, and the messages below will not be printed
     fi
+else
+    echo "Existing stamp not found, running cargo clean"
+    cargo clean
 fi
 
+# If `cargo clean` was run above, then the target/ directory is now
+# gone and the messages below will not be printed
+
 stamp_file=target/rustc_version_stamp
 current_version=$(rustc --version)