Fix 764 (#2093)

* Fix KeyExpiration when a zero time value has a timezone

When a zero time value is loaded from JSON or a DB in a way that
assigns it the local timezone, it does not roudtrip in JSON as a
value for which IsZero returns true. This causes KeyExpiry to be
treated as a far past value instead of a nilish value.

See https://github.com/golang/go/issues/57040

* Fix whitespace

* Ensure that postgresql is used for all tests when env var is set

* Pass through value of HEADSCALE_INTEGRATION_POSTGRES env var

* Add option to set timezone on headscale container

* Add test for registration with auth key in alternate timezone

integration/hsic/hsic.go

@@ -211,6 +211,12 @@ func WithTuning(batchTimeout time.Duration, mapSessionChanSize int) Option {
 	}
 }
 
+func WithTimezone(timezone string) Option {
+	return func(hsic *HeadscaleInContainer) {
+		hsic.env["TZ"] = timezone
+	}
+}
+
 // New returns a new HeadscaleInContainer instance.
 func New(
 	pool *dockertest.Pool,