types.singleLineStr: Allow and trim trailing \n

Allow a \n character at the end of the string and remove it during the
merge function.

An option of this type will resolve to the value "foo" whether it is set
to "foo" or "foo\n".

This is useful when using 'builtins.readFile' or ''-strings, which might
add an unintended newline (for example, bash trim the final newline from
a subshell).

lib/types.nix

@@ -300,11 +300,18 @@ rec {
       inherit (str) merge;
     };
 
-    singleLineStr = mkOptionType {
-      name = "singleLineStr";
-      description = "string that doesn't contain [\\n\\r]";
-      inherit (strMatching "[^\n\r]*") check merge;
-    };
+    # Allow a newline character at the end and trim it in the merge function.
+    singleLineStr =
+      let
+        inherit (strMatching "[^\n\r]*\n?") check merge;
+      in
+      mkOptionType {
+        name = "singleLineStr";
+        description = "string that doesn't contain [\\n\\r]";
+        inherit check;
+        merge = loc: defs:
+          lib.removeSuffix "\n" (merge loc defs);
+      };
 
     strMatching = pattern: mkOptionType {
       name = "strMatching ${escapeNixString pattern}";