Fix install script's check for previous installation (#1603)

The installation script checks for an existing installation in the PATH in order to provide appropriate advice to the
user about adding the installation to their their PATH environment variable.

This check is done using `command -v`. It turns out that the exit status is shell dependent in the event the command is
not found, so that it might be either 1 or 127 depending on the user's system. The script previously assumed that the
exit status would be 1 when the command was not found in PATH, which resulted in spurious advice under these conditions:

```
An existing arduino-cli was found at . Please prepend "/home/foo/arduino-cli/bin" to your $PATH or remove the existing one.
```

It seems safest to fix this by inverting the logic so that the advice about an existing installation in PATH is only
printed when one was found.

Author: per1234

Diff for: install.sh

@@ -186,19 +186,19 @@ bye() {
 
 testVersion() {
   set +e
-  EXECUTABLE_PATH="$(command -v $PROJECT_NAME)"
-  if [ "$?" = "1" ]; then
-    # $PATH is intentionally a literal in this message.
-    # shellcheck disable=SC2016
-    echo "$PROJECT_NAME not found. You might want to add \"$EFFECTIVE_BINDIR\" to your "'$PATH'
-  else
+  if EXECUTABLE_PATH="$(command -v $PROJECT_NAME)"; then
     # Convert to resolved, absolute paths before comparison
     EXECUTABLE_REALPATH="$(cd -- "$(dirname -- "$EXECUTABLE_PATH")" && pwd -P)"
     EFFECTIVE_BINDIR_REALPATH="$(cd -- "$EFFECTIVE_BINDIR" && pwd -P)"
     if [ "$EXECUTABLE_REALPATH" != "$EFFECTIVE_BINDIR_REALPATH" ]; then
+      # $PATH is intentionally a literal in this message.
       # shellcheck disable=SC2016
       echo "An existing $PROJECT_NAME was found at $EXECUTABLE_PATH. Please prepend \"$EFFECTIVE_BINDIR\" to your "'$PATH'" or remove the existing one."
     fi
+  else
+    # $PATH is intentionally a literal in this message.
+    # shellcheck disable=SC2016
+    echo "$PROJECT_NAME not found. You might want to add \"$EFFECTIVE_BINDIR\" to your "'$PATH'
   fi
 
   set -e