feat: check for pre-installed dagger

[jpadams]

closes #164

[jpadams]

Have to decide what to do if user wants to override version of pre-installed dagger. Do we install the desired dagger CLI version in the <install location prefix>/bin if set, or else just install in the default location? or intentionally clobber the existing version in its location (prob not), or install in either the default or designated spot and then make an adjustment to the PATH variable that will last for this actions invocation to give the new version precedence (and/or find a way to persist the PATH variable), or uninstall the previous version (prob not), or ...