standalone: support relative cache path

Author: jacobly0

test/standalone/dirname/build.zig

@@ -55,7 +55,7 @@ pub fn build(b: *std.Build) void {
     const abs_path = setup_abspath: {
         const temp_dir = b.makeTempPath();
 
-        var dir = std.fs.openDirAbsolute(temp_dir, .{}) catch @panic("failed to open temp dir");
+        var dir = std.fs.cwd().openDir(temp_dir, .{}) catch @panic("failed to open temp dir");
         defer dir.close();
 
         var file = dir.createFile("foo.txt", .{}) catch @panic("failed to create file");