[New] install.sh: allow user to explicitly opt out of nvm adding the source string

this is done by checking if the user supplies `PROFILE=/dev/null` when running `install.sh`, the `nvm_detect_profile` function will not output any strings, causing `nvm_do_install` to skip adding `SOURCE_STR`.

install.sh

@@ -212,6 +212,11 @@ nvm_try_profile() {
 # Otherwise, an empty string is returned
 #
 nvm_detect_profile() {
+  if [ "${PROFILE-}" = '/dev/null' ]; then
+    # the user has specifically requested NOT to have nvm touch their profile
+    return
+  fi
+
   if [ -n "${PROFILE}" ] && [ -f "${PROFILE}" ]; then
     echo "${PROFILE}"
     return