Artifacts: Change Dict{AbstractPlatform} to Dict{Platform} to reduce Pkg invalidations (#54073)

This pull request now only changes `dl_dict =
Dict{AbstractPlatform,Dict{String,Any}}()` to `dl_dict =
Dict{Platform,Dict{String,Any}}()` in `artifact_meta`.

This is possible since the other possible types for the key are either
`Platform` or `nothing` unless someone overrides
`Artifacts.unpack_platform`.

(cherry picked from commit a14cc38)

Author: mkitti

stdlib/Artifacts/src/Artifacts.jl

@@ -9,7 +9,9 @@ that would be convenient to place within an immutable, life-cycled datastore.
 module Artifacts
 
 import Base: get, SHA1
-using Base.BinaryPlatforms, Base.TOML
+using Base.BinaryPlatforms: AbstractPlatform, Platform, HostPlatform
+using Base.BinaryPlatforms: tags, triplet, select_platform
+using Base.TOML: TOML
 
 export artifact_exists, artifact_path, artifact_meta, artifact_hash,
        select_downloadable_artifacts, find_artifacts_toml, @artifact_str
@@ -400,7 +402,7 @@ function artifact_meta(name::String, artifact_dict::Dict, artifacts_toml::String
 
     # If it's an array, find the entry that best matches our current platform
     if isa(meta, Vector)
-        dl_dict = Dict{AbstractPlatform,Dict{String,Any}}()
+        dl_dict = Dict{Platform,Dict{String,Any}}()
         for x in meta
             x = x::Dict{String, Any}
             dl_dict[unpack_platform(x, name, artifacts_toml)] = x