Avoid new "out of disk space" issues in CI

Our 1.63 build on Ubuntu has been failing for quite some time
because it runs out of disk space trying to build tests in the last
cfg-flag steps. Thus, we add a few new `cargo clean`s here to fix
it.

Author: TheBlueMatt

ci/ci-tests.sh

@@ -173,5 +173,7 @@ fi
 
 echo -e "\n\nTest cfg-flag builds"
 RUSTFLAGS="--cfg=taproot" cargo test --verbose --color always -p lightning
+[ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
 RUSTFLAGS="--cfg=async_signing" cargo test --verbose --color always -p lightning
+[ "$CI_MINIMIZE_DISK_USAGE" != "" ] && cargo clean
 RUSTFLAGS="--cfg=dual_funding" cargo test --verbose --color always -p lightning