Check if tput command exists before using it

Fix #8311

@eerhardt

scripts/obtain/dotnet-install.sh

@@ -22,7 +22,7 @@ exec 3>&1
 
 # Setup some colors to use. These need to work in fairly limited shells, like the Ubuntu Docker container where there are only 8 colors.
 # See if stdout is a terminal
-if [ -t 1 ]; then
+if [ -t 1 ] && command -v tput > /dev/null; then
     # see if it supports colors
     ncolors=$(tput colors)
     if [ -n "$ncolors" ] && [ $ncolors -ge 8 ]; then