fix: default to GNU host in Cygwin/MSYS/MinGW environments (#4221)

Author: cachebag

rustup-init.sh

@@ -179,6 +179,21 @@ main() {
 
         ignore "$_file" "$@" < /dev/tty
     else
+        # Default to GNU toolchain under Cygwin/MSYS/MinGW environments
+        case "$(uname -s)" in 
+            CYGWIN*|MSYS*|MINGW*)
+                _has_default_host_arg=false
+                for arg in "$@"; do 
+                    case "$arg" in 
+                        --default-host*) _has_default_host_arg=true ;;
+                    esac 
+                done 
+                if [ "$_has_default_host_arg" = false ]; then 
+                    set -- "$@" --default-host x86_64-pc-windows-gnu
+                fi
+                ;;
+        esac 
+
         ignore "$_file" "$@"
     fi