Switch python command to use python3.8

The "python" command was recently removed from the buildbots. Switch to
python3.8, which is the version required at
https://fuchsia.dev/fuchsia-src/contribute/governance/rfcs/0129_python_in_fuchsia?hl=en#implementation

Change-Id: Idd7d47f3ec8d03d0565b054a01887c80e66a8872
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vulkan-cts/+/729287
Reviewed-by: Craig Stout <cstout@google.com>
diff --git a/fuchsia/build.sh b/fuchsia/build.sh
index a44f336..cc3194e 100755
--- a/fuchsia/build.sh
+++ b/fuchsia/build.sh
@@ -41,7 +41,7 @@
 export PATH="$PATH:$(dirname $cmake)"
 
 pushd $fuchsia_root/third_party/vulkan-cts
-python external/fetch_sources.py
+python3.8 external/fetch_sources.py
 popd
 
 mkdir -p $build_dir