Assert that _artifact_str returns a String (#38975)

I have no idea whether this is important, but I noticed this while investigating triggers for inference in a package. Feel free to close if you think this is irrelevant.

Incidentally, `_artifact_str` is a ~60ms precompile (that's just the inference, not the codegen). I see that its precompile was commented out in 924484f; just mentioning it in case there's an opportunity to squeeze out a bit more latency-reduction.

(cherry picked from commit ccaaf56)

stdlib/Artifacts/src/Artifacts.jl

@@ -671,7 +671,7 @@ macro artifact_str(name, platform=nothing)
         platform = HostPlatform()
         artifact_name, artifact_path_tail, hash = artifact_slash_lookup(name, artifact_dict, artifacts_toml, platform)
         return quote
-            Base.invokelatest(_artifact_str, $(__module__), $(artifacts_toml), $(artifact_name), $(artifact_path_tail), $(artifact_dict), $(hash), $(platform), $(lazyartifacts))
+            Base.invokelatest(_artifact_str, $(__module__), $(artifacts_toml), $(artifact_name), $(artifact_path_tail), $(artifact_dict), $(hash), $(platform), $(lazyartifacts))::String
         end
     else
         if platform === nothing
@@ -680,7 +680,7 @@ macro artifact_str(name, platform=nothing)
         return quote
             local platform = $(esc(platform))
             local artifact_name, artifact_path_tail, hash = artifact_slash_lookup($(esc(name)), $(artifact_dict), $(artifacts_toml), platform)
-            Base.invokelatest(_artifact_str, $(__module__), $(artifacts_toml), artifact_name, artifact_path_tail, $(artifact_dict), hash, platform, $(lazyartifacts))
+            Base.invokelatest(_artifact_str, $(__module__), $(artifacts_toml), artifact_name, artifact_path_tail, $(artifact_dict), hash, platform, $(lazyartifacts))::String
         end
     end
 end