fix(make): guard toolchain uninstall on non-empty TOOLCHAIN_NAME

TOOLCHAIN_NAME defaults to empty in the Makefile, which meant
make clean would always run rustup toolchain uninstall "" and fail.
Now we only attempt the uninstall when the variable is actually set.

Author: cds-amal

Makefile

@@ -33,7 +33,7 @@ clean: rustup-clear-toolchain clean-graphs
 rustup-clear-toolchain:
 	rustup override unset
 	rustup override unset --nonexistent
-	rustup toolchain uninstall "$(TOOLCHAIN_NAME)"
+	if [ -n "$(TOOLCHAIN_NAME)" ]; then rustup toolchain uninstall "$(TOOLCHAIN_NAME)"; fi
 
 ### Test