git: Fix the update.sh script and use HTTPS

The syntax is ${parameter:-word} (i.e. previously this used
"latestTag" instead of the actual value).
(Fixes a regression from #85278.)

Also: Even though getting the latest tag isn't really security critical
(as long as Git itself is secure against untrusted input), I'd prefer to
switch from the Git to the HTTPS protocol (for authentication of the
server and encryption + uses a standard port).