cmd/cue: store token expiry timestamps in UTC in logins.json

That is, rather than a local-timezone timestamp such as

    "expiry": "2024-10-01T12:25:11.136734393+02:00"

it is more consistent and straightforward to use UTC:

    "expiry": "2024-10-01T10:25:26.177186044Z"

There should be zero change in the token refresh logic either way,
but this will be easier for humans to grasp and debug.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I673482b6c667eb4bea2f2d6c426ba077142e008a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202036
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cmd/cue/cmd/login.go

@@ -97,6 +97,9 @@ inside $CUE_CONFIG_DIR; see 'cue help environment'.
 				return fmt.Errorf("cannot obtain the OAuth2 token: %v", err)
 			}
 
+			// For consistency, store timestamps in UTC.
+			tok.Expiry = tok.Expiry.UTC()
+
 			_, err = cueconfig.UpdateRegistryLogin(loginsPath, host.Name, tok)
 
 			if err != nil {

cmd/cue/cmd/testdata/script/login_immediate.txtar

@@ -14,7 +14,7 @@ grep -count=1 '"access_token"' cueconfig/logins.json
 # Ensure the contents of the token look correct.
 grep -count=1 '"access_token": "secret-access-token"' cueconfig/logins.json
 grep -count=1 '"token_type": "Bearer"' cueconfig/logins.json
-# TODO(mvdan): store expiry timestamps in UTC for consistency.
-grep '"expiry": "20..-..-..T' cueconfig/logins.json
+# Timestamps are always stored in UTC.
+grep '"expiry": "20..-..-..T.*Z"' cueconfig/logins.json
 # oauthregistry does not give a refresh token, and we use encoding/json's omitempty.
 ! grep '"refresh_token"' cueconfig/logins.json