Fixes two issues with the install script: (#2027)
Fixes two issues with the install script: (#2027)

- Old versions of Mac OS (10.10 and below) require the `-t` option for `mktemp`
 - The check for GPG wasn't returning correctly, and tried to run GPG even when not available

Closes #2012
Closes #1984
lawliet89/yarn,suitupalex/yarn,onemen/yarn,mgcrea/yarn,mvestergaard/yarn,dguo/yarn,seripap/emojiless-yarn,andreypopp/esy,kasperlewau/yarn,ConAntonakos/yarn,bsansouci/esy,mourner/yarn,CHECK24/yarn,mourner/yarn,mvestergaard/yarn,kasperlewau/yarn,ConAntonakos/yarn,lawliet89/yarn,seripap/emojiless-yarn,suitupalex/yarn,mgcrea/yarn,joeyespo/yarn,bsansouci/esy,dguo/yarn,CHECK24/yarn,andreypopp/esy,andreypopp/esy,bsansouci/esy,joeyespo/yarn,onemen/yarn