Fix build.jl: strip() returns SubString, release_url wants String

runtime_tag() fed release_url a SubString{String} from strip(), which
doesn't match its ::String parameter type -- MethodError on any fresh
Pkg.build. Wrap in String(...).

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

Author: joa-quim

deps/build.jl

@@ -23,7 +23,7 @@ const MARKER    = joinpath(BUILD_DIR, ".full_runtime_installed")
 function runtime_tag()
     f = joinpath(DEPS_DIR, "RUNTIME_VERSION")
     isfile(f) || error("deps/RUNTIME_VERSION missing — can't tell which runtime release to fetch")
-    strip(read(f, String))
+    String(strip(read(f, String)))
 end
 
 release_url(tag::String, asset::String) =