Fix @artifact_str invocation

We must provide the `platform` argument here before we provide the
`artifacts_toml_path` argument.

Author: staticfloat

stdlib/Artifacts/test/runtests.jl

@@ -91,7 +91,7 @@ end
         HelloWorldC_exe_path = joinpath(HelloWorldC_dir, "bin", "hello_world$(exeext)")
         @test isfile(HelloWorldC_exe_path)
 
-        HelloWorldC_dir_explicit_artifact = eval(:(@artifact_str "HelloWorldC" joinpath(@__DIR__, "Artifacts.toml")))
+        HelloWorldC_dir_explicit_artifact = eval(:(@artifact_str "HelloWorldC" nothing joinpath(@__DIR__, "Artifacts.toml")))
         @test isdir(HelloWorldC_dir_explicit_artifact)
 
         # Simple slash-indexed lookup