deb: export DEB_RULES_REQUIRES_ROOT for the rules

dpkg-buildpackage normally exports the rules' root requirement from
debian/control; running the rules directly does not, and tools key
their assembly strategy on it - without it debputy refuses to
assemble packages shipping root-owned paths at all (apache2). Policy
defaults to 'binary-targets' when control does not declare it.

src/deb/local.rs

@@ -446,6 +446,28 @@ pub async fn build(
         }
     }
 
+    // dpkg-buildpackage normally exports the rules' root requirement from
+    // debian/control; running the rules directly does not, and tools key
+    // their assembly strategy on it — without it debputy refuses to
+    // assemble packages shipping root-owned paths at all (apache2). Policy
+    // defaults to 'binary-targets' when control does not declare it.
+    let control = ctx
+        .command("cat")
+        .current_dir(package_dir_str)
+        .arg("debian/control")
+        .output()
+        .map_err(|e| format!("cannot read debian/control: {e}"))?;
+    let rules_requires_root = String::from_utf8_lossy(&control.stdout)
+        .lines()
+        .find(|line| {
+            line.to_ascii_lowercase()
+                .starts_with("rules-requires-root:")
+        })
+        .and_then(|line| line.split_once(':'))
+        .map(|(_, value)| value.trim().to_string())
+        .unwrap_or_else(|| "binary-targets".to_string());
+    env.insert("DEB_RULES_REQUIRES_ROOT".to_string(), rules_requires_root);
+
     // Run the build step
     log::debug!("Building (debian/rules build) package...");
     enter_phase(view, Phase::Building);