Replace the build job with the build-appleclang job.

Unlike GCC and upstream Clang, AppleClang still defaults to `-std=c++98`
for some reason. Also, the macOS image on GitHub Actions provides wildly
numbered Xcode versions. Thus, rather than varying the compiler version,
we set the `-std` flag explicitly in order to vary the language version.

Change-Id: I662118caea086b2d46ffb0d4283d019c13a40a9d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59650
Reviewed-by: Emil Mikulic <easy@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed