[libaom] Run cmake in a subdirectory under $WORK. (#1625)
[libaom] Run cmake in a subdirectory under $WORK. (#1625)

Before we run cmake, we remove everything in the directory where we run
cmake. This will remove /work/msan if we run cmake in $WORK. To avoid
that, create a $WORK/build directory and run cmake in that directory.
robertswiecki/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,FeliciaLim/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,FeliciaLim/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,FeliciaLim/oss-fuzz,google/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,FeliciaLim/oss-fuzz,skia-dev/oss-fuzz,FeliciaLim/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz