fix: avoid rebuilding libsql-ffi every time when using Multiple Ciphers (#2178)

Fix https://github.com/tursodatabase/libsql/issues/2177

When using the Multiple Ciphers extension in libsql, I noticed that it
was rebuilding libsql-ffi every time. This fixes the condition in
`build.rs` to only check for changes in the Multiple Ciphers source