lib/strings: Zero‐pad hex digits in escapeC (#399722)

lib/strings.nix

@@ -998,7 +998,11 @@ rec {
 
     :::
   */
-  escapeC = list: replaceStrings list (map (c: "\\x${toLower (lib.toHexString (charToInt c))}") list);
+  escapeC =
+    list:
+    replaceStrings list (
+      map (c: "\\x${fixedWidthString 2 "0" (toLower (lib.toHexString (charToInt c)))}") list
+    );
 
   /**
     Escape the `string` so it can be safely placed inside a URL

lib/tests/misc.nix

@@ -851,8 +851,8 @@ runTests {
   };
 
   testEscapeC = {
-    expr = strings.escapeC [ " " ] "Hello World";
-    expected = "Hello\\x20World";
+    expr = strings.escapeC [ "\n" " " ] "Hello World\n";
+    expected = "Hello\\x20World\\x0a";
   };
 
   testEscapeURL = testAllTrue [