Select appropriate timeout for each Github runner

Testing shows 600 seconds is needed to boot Linux on macOS runners,
while 90 seconds works for Linux runners. This commit sets the timeout
for each platform accordingly.

The varying timeout requirements will be tracked in an issue.

.ci/autorun.sh

@@ -16,8 +16,15 @@ function ASSERT {
 
 cleanup
 
+UNAME_S=$(uname -s)
+if [[ ${UNAME_S} == "Darwin" ]]; then
+    TIMEOUT=600
+else # Linux
+    TIMEOUT=90
+fi
+
 ASSERT expect <<DONE
-set timeout 90
+set timeout ${TIMEOUT}
 spawn make check
 expect "buildroot login:" { send "root\n" } timeout { exit 1 }
 expect "# " { send "uname -a\n" } timeout { exit 2 }