fix: [torrust#56] triggering recompilation on migration changes

Migrations were not executed while running integration tests.
After adding a new migration without changing any Rust code.

More info: https://docs.rs/sqlx/latest/sqlx/macro.migrate.html#triggering-recompilation-on-migration-changes

build.rs

@@ -0,0 +1,5 @@
+// generated by `sqlx migrate build-script`
+fn main() {
+    // trigger recompilation when a new migration is added
+    println!("cargo:rerun-if-changed=migrations");
+}