Check exit status directly in install script

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 by checking the exit status of `command -v`. The previous use of `$?` for that check was unnecessary
since the command itself can be checked.

This change is required for compliance with ShellCheck rule SC2181:

https://github.com/koalaman/shellcheck/wiki/SC2181

Author: per1234

etc/install.sh

@@ -186,8 +186,7 @@ bye() {
 
 testVersion() {
   set +e
-  EXECUTABLE_PATH="$(command -v $PROJECT_NAME)"
-  if [ "$?" = "0" ]; then
+  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)"