`install.sh`: detect a defined but nonexistent $NVM_DIR
`install.sh`: detect a defined but nonexistent $NVM_DIR

(ref: #1700)

creationix/nvm,evocateur/nvm,creationix/nvm,ljharb/nvm,evocateur/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm,PeterDaveHello/nvm,PeterDaveHelloKitchen/nvm,edwmurph/nvm,edwmurph/nvm,ljharb/nvm