vcomp/lavc: fix default VP9 encoder for non-avx2

the default - libsvt_vp9 - requires AVX2 instructions so if not present,
use the fallback.

broken by some weird mistake since the commit ff3e34e (24th Oct 2025)

Author: MartinPulec

src/video_compress/libavcodec.cpp

@@ -200,7 +200,7 @@ static void cleanup(struct state_video_compress_libav *s);
 
 const char *get_vp9_encoder(bool /* rgb */) {
 #ifdef __x86_64__
-        return __builtin_cpu_supports("avx2") ? nullptr : "libaom-av1";
+        return __builtin_cpu_supports("avx2") ? nullptr : "libvpx-vp9";
 #else
         return nullptr;
 #endif