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/unix/build/derivation-builder.cc

@@ -46,7 +46,7 @@
 #include "store-config-private.hh"
 #include "build/derivation-check.hh"
 
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
 #  include "nix/store/aws-creds.hh"
 #  include "nix/store/s3-url.hh"
 #  include "nix/util/url.hh"
@@ -296,7 +296,7 @@ protected:
      */
     virtual void startChild();
 
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
     /**
      * Pre-resolve AWS credentials for S3 URLs in builtin:fetchurl.
      * This should be called before forking to ensure credentials are available in child.
@@ -359,7 +359,7 @@ protected:
      */
     struct RunChildArgs
     {
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
         std::optional<AwsCredentials> awsCredentials;
 #endif
     };
@@ -945,7 +945,7 @@ void DerivationBuilderImpl::openSlave()
         throw SysError("cannot pipe standard error into log file");
 }
 
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
 std::optional<AwsCredentials> DerivationBuilderImpl::preResolveAwsCredentials()
 {
     if (drv.isBuiltin() && drv.builder == "builtin:fetchurl") {
@@ -974,7 +974,7 @@ std::optional<AwsCredentials> DerivationBuilderImpl::preResolveAwsCredentials()
 void DerivationBuilderImpl::startChild()
 {
     RunChildArgs args{
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
         .awsCredentials = preResolveAwsCredentials(),
 #endif
     };
@@ -1255,7 +1255,7 @@ void DerivationBuilderImpl::runChild(RunChildArgs args)
         BuiltinBuilderContext ctx{
             .drv = drv,
             .tmpDirInSandbox = tmpDirInSandbox(),
-#if NIX_WITH_S3_SUPPORT
+#if NIX_WITH_AWS_AUTH
             .awsCredentials = args.awsCredentials,
 #endif
         };

src/libstore/unix/build/linux-derivation-builder.cc

@@ -277,7 +277,7 @@ struct ChrootLinuxDerivationBuilder : ChrootDerivationBuilder, LinuxDerivationBu
     void startChild() override
     {
         RunChildArgs args{
-#  if NIX_WITH_S3_SUPPORT
+#  if NIX_WITH_AWS_AUTH
             .awsCredentials = preResolveAwsCredentials(),
 #  endif
         };