chore: Makevars templates results in _abort note

[JosiahParry]

The recent changes to the Makevars template results in the _abort note being present again.

This will cause failing CRAN checks. To fix this we need to adjust the steps in the Makevars.in templates:

all: $(SHLIB) rust_clean
-    rm -f $(CURDIR)/Makevars.win
clean:
-    rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS) $(TARGET_DIR) $(VENDOR_DIR)
+    rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS) $(TARGET_DIR)

Before: https://github.com/extendr/b64/actions/runs/15119882848/job/42499543460#step:6:115
After: https://github.com/extendr/b64/actions/runs/15120085055/job/42500210905#step:6:124

This should definitely be addressed before a CRAN release.

[JosiahParry]

This is resolved.