Update actions/checkout in GitHub Actions to v4

[striezel]

This updates actions/checkout in the GitHub Actions workflows to v4, it's current version.

Changelog:

v4.1.0

• Add support for partial checkout filters

v4.0.0

• Support fetching without the --progress option
• Update to node20

[striezel]

Not sure why the GCC 10 and 11 builds fail, but the error message is:

 /home/runner/work/atomic-lite/atomic-lite/test/atomic.t.cpp: In function ‘void f(int)’:
/home/runner/work/atomic-lite/atomic-lite/test/atomic.t.cpp:58:5: error: ‘_mm_pause’ was not declared in this scope
   58 |     _mm_pause();
      |     ^~~~~~~~~

They also failed in the previous builds in December 2022, but I can't say whether it was for the same reason, because the logs for thos older build jobs have expired. :|

[striezel]

@martinmoene: Tests are passing now. It looks like #include <immintrin.h> is required to use the _mm_pause() function. I've just added that. (I got the hint from this page of the GCC online documentation which mentions it in the code example.)