Support fetch-test-deps.sh --get-deps debian

Also use `apt-get` instead of `pip` to install
Pillow for libbet

test/fetch-test-deps.sh

@@ -57,12 +57,12 @@ case "$actionname" in
 			# libbet depends on PIL to build
 			if [ "$2" = "libbet" ]; then
 				case "${osname%-*}" in
+					ubuntu | debian)
+						sudo apt-get install python3-pil
+						;;
 					macos)
 						python3 -m pip install --break-system-packages pillow
 						;;
-					ubuntu)
-						python3 -m pip install pillow
-						;;
 					windows)
 						py -3 -m pip install pillow
 						;;