fix(stdexec): skip inline_scheduler's static_assert under nvcc

[alwaysprince05]

Problem

Including <stdexec/execution.hpp> fails to compile under nvcc (#2284):

error: 'struct stdexec::inline_scheduler::__sender' is private within this context

The namespace-scope static_assert at the bottom of __inline_scheduler.hpp names the
private nested __sender type via schedule_result_t<inline_scheduler>, and nvcc's
parse-time evaluation doesn't see through the access the way host compilers do.

Fix

Guard the assert with #if !defined(__CUDACC__), as suggested in #2284. The check
is a compile-time sanity check with no runtime effect; host compilers see no change.
I used the raw macro rather than STDEXEC_CUDA_COMPILATION() to keep the guard as
narrow as the reported failure — happy to switch if the wider macro is preferred.

fixes #2284

Verification

• Preprocessor check: the assert is present in host compilation and absent when
  CUDACC is defined.
• test.stdexec + test.exec rebuilt and pass: 100% of 993 tests, in both the normal
  build and with STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON.
• Minimal repro compiles clean with -Wall -Wextra -Werror on Apple Clang 21.
• clang-format --dry-run --Werror passes on the touched file.
• No nvcc on my machine, so the nvcc compile itself still needs a run on a CUDA
  toolchain (CI or a reviewer).

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[alwaysprince05]

/ok to test