Remove standalone clang from PATH

[wantehchang]

Use the clang in Visual Studio. Recommended in
actions/runner-images#10001 (comment).

[wantehchang]

@vrabaud Vincent: I can't reproduce the avifcodectest failure on my Windows laptop. I use the clang-cl in Visual Studio, and this pull request emulates my setup. The only difference is that I disable the rav1e codec (because I don't have Rust installed).

[vrabaud]

We can also wait for clang 18 to be in by default but your solution is more futureproof so LGTM

[wantehchang]

I abandoned this pull request. After 02816a3, we are not compiling libaom with clang-cl. Although we are still compiling libyuv with clang-cl, libyuv doesn't use the C++ STL. (libyuv is essentially C code compiled with C++ compilers.) So we no longer need this workaround.

Although it is arguably safer to use the clang bundled with Visual Studio, I think it is good to test libavif with a standalone installation of clang.