aom.cmd: Use version on libaom 3.7 release branch

The libaom 3.7 release branch is more stable than the main branch. Will
switch to the v3.7.1 release tag when it is available.

ext/aom.cmd

@@ -8,10 +8,10 @@
 : # If you're running this on Windows, be sure you've already run this (from your VC2019 install dir):
 : #     "C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Auxiliary\Build\vcvars64.bat"
 
-git clone --single-branch https://aomedia.googlesource.com/aom
+git clone -b ironbark --single-branch https://aomedia.googlesource.com/aom
 
 cd aom
-git checkout 2603b3a2d63ba6b1265cde65316950ad301966aa
+git checkout 67d97ee720d57b71c23e500007d8c54577615b89
 
 mkdir build.libavif
 cd build.libavif