chore(mpp): update CMake install prefix to use FFBUILD_PREFIX variable

* Changed the `-DCMAKE_INSTALL_PREFIX` from a hardcoded path to use the `"$FFBUILD_PREFIX"` variable for better flexibility in build configurations.

Author: BenLocal

scripts.d/51-rk/50-mpp.sh

@@ -14,7 +14,7 @@ ffbuild_dockerdl() {
 ffbuild_dockerbuild() {
     mkdir build && cd build
     cmake \
-        -DCMAKE_INSTALL_PREFIX=/usr \
+        -DCMAKE_INSTALL_PREFIX="$FFBUILD_PREFIX" \
         -DCMAKE_BUILD_TYPE=Release \
         -DBUILD_SHARED_LIBS=ON \
         -DBUILD_TEST=OFF