Use absolute path for libajantv2 library.
The relative path was used as for now, which is true in the CI (libajantv2
is extracted to .) but is less eligible for interactive use.
Built libajantv2 has now a different name:
```
2024-07-25T10:26:49.7807780Z Creating library D:/a/UltraGrid/UltraGrid/libajantv2/build/ajantv2/Release/ajantv2_vs143_MT.lib and object D:/a/UltraGrid/UltraGrid/libajantv2/build/ajantv2/Release/ajantv2_vs143_MT.exp
2024-07-25T10:26:50.0519395Z ajantv2.vcxproj -> D:\a\UltraGrid\UltraGrid\libajantv2\build\ajantv2\Release\ajantv2_vs143_MT.dll
```
Since we must now build the new libajantv2, which takes some 3 minutes,
cache the build in order not to increase the build time.
Also use libajantv2 path for the SDK instead of AJA (doesn't require
unneeded rename), which will be Windows specific.
Only do it in MSW, since the build is slowest. But it can be easily used
also for other platforms.
Use libajantv2 + switch the build system to the new lib at the same time
(it is easier and there is perhaps not a big advantage to splitthis to
2 steps).
