refactor(libstore): rename NIX_WITH_S3_SUPPORT to NIX_WITH_AWS_AUTH

The macro now accurately reflects its purpose: gating only AWS
authentication code, not all S3 functionality. S3 URL parsing, store
configuration, and public bucket access work regardless of this flag.

This rename clarifies that:
- S3 support is always available (URL parsing, store registration)
- Only AWS credential resolution requires the flag
- The flag controls AWS CRT SDK dependency, not S3 protocol support

src/libstore/builtins/fetchurl.cc

@@ -41,7 +41,7 @@ static void builtinFetchurl(const BuiltinBuilderContext & ctx)
             FileTransferRequest request(VerbatimURL{url});
             request.decompress = false;
 
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
             // Use pre-resolved credentials if available
             if (ctx.awsCredentials && request.uri.scheme() == "s3") {
                 debug("[pid=%d] Using pre-resolved AWS credentials from parent process", getpid());