build.zig: fix building raylib for emscripten

build.zig doesn't support the examples for the emscripten OS but it does
support the raylib library. However, the examples panic if emscripten
is configured which prevents it from building the library. I've replaced
this panic with a fail step to fix this.

build.zig

@@ -504,7 +504,7 @@ fn addExamples(
     raylib: *std.Build.Step.Compile,
 ) !*std.Build.Step {
     if (target.result.os.tag == .emscripten) {
-        @panic("Emscripten building via Zig unsupported");
+        return &b.addFail("Emscripten building via Zig unsupported").step;
     }
 
     const all = b.step(module, "All " ++ module ++ " examples");