tests/fate/ffmpeg: evaluate thread count in fate-run.sh rather than make

Fixes fate-ffmpeg-loopback-decoding with THREADS=random*

tests/fate/ffmpeg.mak

@@ -259,5 +259,5 @@ FATE_FFMPEG-$(call REMUX, RAWVIDEO) += fate-ffmpeg-streamcopy-t
 fate-ffmpeg-loopback-decoding: tests/data/vsynth_lena.yuv
 fate-ffmpeg-loopback-decoding: CMD = transcode \
     "rawvideo -s 352x288 -pix_fmt yuv420p" $(TARGET_PATH)/tests/data/vsynth_lena.yuv nut \
-    "-map 0:v:0 -c:v mpeg2video -f null - -flags +bitexact -idct simple -threads $(THREADS) -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' -map '[stack]' -c:v ffv1" ""
+    "-map 0:v:0 -c:v mpeg2video -f null - -flags +bitexact -idct simple -threads $$threads -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' -map '[stack]' -c:v ffv1" ""
 FATE_FFMPEG-$(call ENCDEC2, MPEG2VIDEO, FFV1, NUT, HSTACK_FILTER PIPE_PROTOCOL FRAMECRC_MUXER) += fate-ffmpeg-loopback-decoding