fix(nns-tools): Use gdate command instead of date (#4008)

Both Max and jason were unable to run the changelog-related commands
because their environment was defaulting to the macos `date` command
rather than the gnu one, which is more full-featured.

I think gdate will work for them though. It comes with coreutils which
most people should have already probably (i think you need it to compile
rust?). Also the release runscript will tell you to install coreutils if
you haven't yet.

testnet/tools/nns-tools/add-release-to-changelog.sh

@@ -75,9 +75,9 @@ else
     print_red "(In particular, unable to determine which canister and commit.)" >&2
     exit 1
 fi
-SECONDS_AGO=$(($(date +%s) - "${PROPOSED_TIMESTAMP_SECONDS}"))
+SECONDS_AGO=$(($(gdate +%s) - "${PROPOSED_TIMESTAMP_SECONDS}"))
 PROPOSED_ON=$(
-    date --utc \
+    gdate --utc \
         --date=@"${PROPOSED_TIMESTAMP_SECONDS}" \
         --iso-8601
 )