manually set julia_version in manifest

Author: IanButterworth

src/Prefix.jl

@@ -718,6 +718,12 @@ function setup_dependencies(prefix::Prefix,
         if !isempty(stdlib_pkgspecs)
             # TODO: shouldn't this take platform?
             Pkg_add(stdlib_pkgspecs; io=outs, julia_version=nothing)
+
+            # Pkg.add doesn't record julia_version when nothing, so we need to manually set it
+            # to avoid warnings about the manifest not having a julia version.
+            ctx = Pkg.Types.Context()
+            ctx.env.manifest.julia_version = Pkg.Operations.dropbuild(julia_version)
+            Pkg.write_manifest(ctx.env)
         end
 
         # Load their Artifacts.toml files