Makefile: Remove authentication token validity ldflag hack

In the internal/gitaly/server test suite, we have a test which verifies that tokens are checked before the actual RPC gets triggered. This is done quite roundabout via git hooks: we invoke an RPC of which we know that it triggers hooks, then inside of the hook we sleep for the token validity duration. If the token was checked after hook execution, then it wouldn't have been valid anymore and thus failed the RPC.

There's one problem though: by default, tokens are valid for 30 seconds, and waiting twice for 30 seconds to fly by in a test is really awkwardly long. This is why we currently have a hack in our Makefile to decrease the token validity duration to 5 seconds via a -X linker flag. Despite being quite gross, it also has the issue that using go test directly without invoking the Makefile doesn't change the value.

Until now, nobody actually bothered because in fact the test doesn't work: hooks don't get executed at all and thus the UserCreateTag request finishes quite fast, not hitting the timeout. But if they did, then using go test on it would fail currently as we would sleep for twice the default timeout value, while we have a test timeout of twice the default timeout value. Given that the we not only sleep for that exact amount of time but also perform the actual RPC logic, we'd thus typically hit the timeout and fail the test.

Let's remove this ldflag hack and just use our newly grown auth.SetTokenValidityDuration() function, which makes it word both when using the Makefile and when executing tests directly. Note that this commit doesn't fix hook execution, as this will become a lot easier to do as soon as the Go port of UserCreateTag lands.