Migrate daily builds to UTC (#1417)

Temporary patch to keep the build names predictable. Ideally we wouldn't
need to specify the exact date of the build, but for now this patch
should at least keep it protected from daylight savings timezones

daily-tags.sh

@@ -6,7 +6,7 @@ substitute_vars () {
   value=${value//!!FLPSUITE_LATEST!!/$flpsuite_latest}
   value=${value//!!FLPSUITE_CURRENT!!/$flpsuite_current}
   value=${value//!!ALIDIST_BRANCH!!/$ALIDIST_BRANCH}
-  LANG=C TZ=Europe/Zurich date -d "@$START_TIMESTAMP" "+$value"
+  LANG=C TZ=UTC date -d "@$START_TIMESTAMP" "+$value"
 }
 
 edit_package_tag () {
@@ -157,7 +157,8 @@ EOF
 )
 
 # Finally, replace strftime formatting (%Y, %m, %d etc) in the pattern.
-AUTOTAG_TAG=$(LANG=C TZ=Europe/Zurich date -d "@$START_TIMESTAMP" "+$AUTOTAG_PATTERN")
+# We use the UTC timezone to avoid issues with DST changes.
+AUTOTAG_TAG=$(LANG=C TZ=UTC date -d "@$START_TIMESTAMP" "+$AUTOTAG_PATTERN")
 
 : "${AUTOTAG_TAG:?}"   # make sure the tag isn't empty
 [ "$TEST_TAG" = true ] && AUTOTAG_TAG=TEST-IGNORE-$AUTOTAG_TAG