rollup merge of #21274: estsauver/21270

The timezone of the server that builds rustc nightlies
appears to be in UTC. From what I can tell, it builds
the nightlies at 0300 UTC, which takes about ~15 minutes.
So, there were a couple options here for which offset to use.

UTC+3 would ensure that you always got the latest rust, but it
would mean the script is broken ~15/20 minutes a day. UTC+4
means users get a stale nightly for 45 minutes, but that feels
okay to me.

Ideally, buildbot would publish the "current" nightly, but
that seems unneeded relative to fixing it for all Timezones
quickly.

Fixes #21270

src/etc/rustup.sh

@@ -433,11 +433,16 @@ CFG_TMP_DIR=$(mktemp -d 2>/dev/null \
            || mktemp -d -t 'rustup-tmp-install' 2>/dev/null \
            || create_tmp_dir)
 
-# If we're saving nightlies and we didn't specify which one, grab todays.
-# Otherwise we'll use the latest version.
+# If we're saving nightlies and we didn't specify which one, grab the latest
+# verison from the perspective of the server. Buildbot has typically finished
+# building and uploading by ~8UTC, but we want to include a little buffer.
+#
+# FIXME It would be better to use the known most recent nightly that has been
+# built. This is waiting on a change to have buildbot publish metadata that
+# can be queried.
 if [ -n "${CFG_SAVE}" -a -z "${CFG_DATE}" ];
 then
-    CFG_DATE=`date "+%Y-%m-%d"`
+    CFG_DATE=`TZ=Etc/UTC+9 date "+%Y-%m-%d"`
 fi
 
 RUST_URL="https://static.rust-lang.org/dist"