The s3.hh public header was incorrectly including store-config-private.hh
instead of the public config.hh. Since NIX_WITH_S3_SUPPORT is defined in
the public config, this caused clang-tidy to report it as undefined.

Make lambda capture explicit to avoid clang-analyzer-core.CallAndMessage warning

The default constructor for Attr was not initializing the value pointer,
which could lead to undefined behavior when the uninitialized pointer is
accessed. This was caught by clang-tidy's UninitializedObject check.

This fixes the warning:
  1 uninitialized field at the end of the constructor call
  [clang-analyzer-optin.cplusplus.UninitializedObject]