fix: use build-ffmpeg-builder dir for aom and svtav1 cmake builds

aom's source repo stores its cmake helpers inside build/cmake/ (e.g.
aom_configure.cmake, aom_install.cmake). Using build/ as the cmake
binary dir and then rm -rf'ing it deleted those source files, causing
"include could not find requested file" errors on subsequent runs.

Use build-ffmpeg-builder instead, consistent with the srt and vmaf ports.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: alfg

scripts/_ports/aom.sh

@@ -7,7 +7,9 @@ PAR="$3"
 
 AOM_VERSION="${PORT_AOM_VERSION:-v3.9.0}"
 SRC_DIR="$SRC/aom"
-BUILD_DIR="$SRC_DIR/build"
+# aom's repo contains source cmake helpers at build/cmake/ so we must use a
+# different name to avoid clobbering them when we rm -rf the build directory.
+BUILD_DIR="$SRC_DIR/build-ffmpeg-builder"
 
 mkdir -p "$SRC"
 

scripts/_ports/svtav1.sh

@@ -7,7 +7,7 @@ PAR="$3"
 
 SVTAV1_VERSION="${PORT_SVTAV1_VERSION:-v2.2.1}"
 SRC_DIR="$SRC/SVT-AV1"
-BUILD_DIR="$SRC_DIR/build"
+BUILD_DIR="$SRC_DIR/build-ffmpeg-builder"
 
 mkdir -p "$SRC"