Declare Downloads stdlib dep, fixes deps/build.jl sandbox error

Pkg.build runs deps/build.jl in an isolated environment that only
resolves what's in Project.toml [deps] -- stdlibs aren't free there like
they are inside the package's own module code. Without this, build.jl
failed with "Package Downloads not found in current path" on any fresh
install.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: joa-quim

Project.toml

@@ -5,10 +5,12 @@ version = "0.1.0"
 
 [deps]
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
+Downloads = "f43a241f-c20a-4ad4-852c-f6b1247861c6"
 GMT = "5752ebe1-31b9-557e-87aa-f909b540aa54"
 PrecompileTools = "aea7be01-6a6a-4083-8856-8a6e6704d82a"
 
 [compat]
+Downloads = "1"
 GMT = "1.40"
 julia = "1.10"